











Network Working Group                                 J. Klensin, Editor

Request for Comments: 2821                             AT&T Laboratories

Obsoletes: 821, 974, 1869                                     April 2001

Updates: 1123

Category: Standards Track





                     Simple Mail Transfer Protocol



Status of this Memo



   This document specifies an Internet standards track protocol for the

   Internet community, and requests discussion and suggestions for

   improvements.  Please refer to the current edition of the "Internet

   Official Protocol Standards" (STD 1) for the standardization state

   and status of this protocol.  Distribution of this memo is unlimited.



Copyright Notice



   Copyright (C) The Internet Society (2001).  All Rights Reserved.



Abstract



   This document is a self-contained specification of the basic protocol

   for the Internet electronic mail transport.  It consolidates, updates

   and clarifies, but doesn't add new or change existing functionality

   of the following:



   -  the original SMTP (Simple Mail Transfer Protocol) specification of

      RFC 821 [30],



   -  domain name system requirements and implications for mail

      transport from RFC 1035 [22] and RFC 974 [27],



   -  the clarifications and applicability statements in RFC 1123 [2],

      and



   -  material drawn from the SMTP Extension mechanisms [19].



   It obsoletes RFC 821, RFC 974, and updates RFC 1123 (replaces the

   mail transport materials of RFC 1123).  However, RFC 821 specifies

   some features that were not in significant use in the Internet by the

   mid-1990s and (in appendices) some additional transport models.

   Those sections are omitted here in the interest of clarity and

   brevity; readers needing them should refer to RFC 821.













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   It also includes some additional material from RFC 1123 that required

   amplification.  This material has been identified in multiple ways,

   mostly by tracking flaming on various lists and newsgroups and

   problems of unusual readings or interpretations that have appeared as

   the SMTP extensions have been deployed.  Where this specification

   moves beyond consolidation and actually differs from earlier

   documents, it supersedes them technically as well as textually.



   Although SMTP was designed as a mail transport and delivery protocol,

   this specification also contains information that is important to its

   use as a 'mail submission' protocol, as recommended for POP [3, 26]

   and IMAP [6].  Additional submission issues are discussed in RFC 2476

   [15].



   Section 2.3 provides definitions of terms specific to this document.

   Except when the historical terminology is necessary for clarity, this

   document uses the current 'client' and 'server' terminology to

   identify the sending and receiving SMTP processes, respectively.



   A companion document [32] discusses message headers, message bodies

   and formats and structures for them, and their relationship.



Table of Contents



   1. Introduction ..................................................  4

   2. The SMTP Model ................................................  5

   2.1 Basic Structure ..............................................  5

   2.2 The Extension Model ..........................................  7

   2.2.1 Background .................................................  7

   2.2.2 Definition and Registration of Extensions ..................  8

   2.3 Terminology ..................................................  9

   2.3.1 Mail Objects ............................................... 10

   2.3.2 Senders and Receivers ...................................... 10

   2.3.3 Mail Agents and Message Stores ............................. 10

   2.3.4 Host ....................................................... 11

   2.3.5 Domain ..................................................... 11

   2.3.6 Buffer and State Table ..................................... 11

   2.3.7 Lines ...................................................... 12

   2.3.8 Originator, Delivery, Relay, and Gateway Systems ........... 12

   2.3.9 Message Content and Mail Data .............................. 13

   2.3.10 Mailbox and Address ....................................... 13

   2.3.11 Reply ..................................................... 13

   2.4 General Syntax Principles and Transaction Model .............. 13

   3. The SMTP Procedures: An Overview .............................. 15

   3.1 Session Initiation ........................................... 15

   3.2 Client Initiation ............................................ 16

   3.3 Mail Transactions ............................................ 16

   3.4 Forwarding for Address Correction or Updating ................ 19







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   3.5 Commands for Debugging Addresses ............................. 20

   3.5.1 Overview ................................................... 20

   3.5.2 VRFY Normal Response ....................................... 22

   3.5.3 Meaning of VRFY or EXPN Success Response ................... 22

   3.5.4 Semantics and Applications of EXPN ......................... 23

   3.6 Domains ...................................................... 23

   3.7 Relaying ..................................................... 24

   3.8 Mail Gatewaying .............................................. 25

   3.8.1 Header Fields in Gatewaying ................................ 26

   3.8.2 Received Lines in Gatewaying ............................... 26

   3.8.3 Addresses in Gatewaying .................................... 26

   3.8.4 Other Header Fields in Gatewaying .......................... 27

   3.8.5 Envelopes in Gatewaying .................................... 27

   3.9 Terminating Sessions and Connections ......................... 27

   3.10 Mailing Lists and Aliases ................................... 28

   3.10.1 Alias ..................................................... 28

   3.10.2 List ...................................................... 28

   4. The SMTP Specifications ....................................... 29

   4.1 SMTP Commands ................................................ 29

   4.1.1 Command Semantics and Syntax ............................... 29

   4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO) ................... 29

   4.1.1.2 MAIL (MAIL) .............................................. 31

   4.1.1.3 RECIPIENT (RCPT) ......................................... 31

   4.1.1.4 DATA (DATA) .............................................. 33

   4.1.1.5 RESET (RSET) ............................................. 34

   4.1.1.6 VERIFY (VRFY) ............................................ 35

   4.1.1.7 EXPAND (EXPN) ............................................ 35

   4.1.1.8 HELP (HELP) .............................................. 35

   4.1.1.9 NOOP (NOOP) .............................................. 35

   4.1.1.10 QUIT (QUIT) ............................................. 36

   4.1.2 Command Argument Syntax .................................... 36

   4.1.3 Address Literals ........................................... 38

   4.1.4 Order of Commands .......................................... 39

   4.1.5 Private-use Commands ....................................... 40

   4.2  SMTP Replies ................................................ 40

   4.2.1 Reply Code Severities and Theory ........................... 42

   4.2.2 Reply Codes by Function Groups ............................. 44

   4.2.3  Reply Codes in Numeric Order .............................. 45

   4.2.4 Reply Code 502 ............................................. 46

   4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF> .... 46

   4.3 Sequencing of Commands and Replies ........................... 47

   4.3.1 Sequencing Overview ........................................ 47

   4.3.2 Command-Reply Sequences .................................... 48

   4.4 Trace Information ............................................ 49

   4.5 Additional Implementation Issues ............................. 53

   4.5.1 Minimum Implementation ..................................... 53

   4.5.2 Transparency ............................................... 53

   4.5.3 Sizes and Timeouts ......................................... 54







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   4.5.3.1 Size limits and minimums ................................. 54

   4.5.3.2 Timeouts ................................................. 56

   4.5.4 Retry Strategies ........................................... 57

   4.5.4.1 Sending Strategy ......................................... 58

   4.5.4.2 Receiving Strategy ....................................... 59

   4.5.5 Messages with a null reverse-path .......................... 59

   5. Address Resolution and Mail Handling .......................... 60

   6. Problem Detection and Handling ................................ 62

   6.1 Reliable Delivery and Replies by Email ....................... 62

   6.2 Loop Detection ............................................... 63

   6.3 Compensating for Irregularities .............................. 63

   7. Security Considerations ....................................... 64

   7.1 Mail Security and Spoofing ................................... 64

   7.2 "Blind" Copies ............................................... 65

   7.3 VRFY, EXPN, and Security ..................................... 65

   7.4 Information Disclosure in Announcements ...................... 66

   7.5 Information Disclosure in Trace Fields ....................... 66

   7.6 Information Disclosure in Message Forwarding ................. 67

   7.7 Scope of Operation of SMTP Servers ........................... 67

   8. IANA Considerations ........................................... 67

   9. References .................................................... 68

   10. Editor's Address ............................................. 70

   11. Acknowledgments .............................................. 70

   Appendices ....................................................... 71

   A. TCP Transport Service ......................................... 71

   B. Generating SMTP Commands from RFC 822 Headers ................. 71

   C. Source Routes ................................................. 72

   D. Scenarios ..................................................... 73

   E. Other Gateway Issues .......................................... 76

   F. Deprecated Features of RFC 821 ................................ 76

   Full Copyright Statement ......................................... 79



1. Introduction



   The objective of the Simple Mail Transfer Protocol (SMTP) is to

   transfer mail reliably and efficiently.



   SMTP is independent of the particular transmission subsystem and

   requires only a reliable ordered data stream channel.  While this

   document specifically discusses transport over TCP, other transports

   are possible.  Appendices to RFC 821 describe some of them.



   An important feature of SMTP is its capability to transport mail

   across networks, usually referred to as "SMTP mail relaying" (see

   section 3.8).  A network consists of the mutually-TCP-accessible

   hosts on the public Internet, the mutually-TCP-accessible hosts on a

   firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN

   environment utilizing a non-TCP transport-level protocol.  Using







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   SMTP, a process can transfer mail to another process on the same

   network or to some other network via a relay or gateway process

   accessible to both networks.



   In this way, a mail message may pass through a number of intermediate

   relay or gateway hosts on its path from sender to ultimate recipient.

   The Mail eXchanger mechanisms of the domain name system [22, 27] (and

   section 5 of this document) are used to identify the appropriate

   next-hop destination for a message being transported.



2. The SMTP Model



2.1 Basic Structure



   The SMTP design can be pictured as:



               +----------+                +----------+

   +------+    |          |                |          |

   | User |<-->|          |      SMTP      |          |

   +------+    |  Client- |Commands/Replies| Server-  |

   +------+    |   SMTP   |<-------------->|    SMTP  |    +------+

   | File |<-->|          |    and Mail    |          |<-->| File |

   |System|    |          |                |          |    |System|

   +------+    +----------+                +----------+    +------+

                SMTP client                SMTP server



   When an SMTP client has a message to transmit, it establishes a two-

   way transmission channel to an SMTP server.  The responsibility of an

   SMTP client is to transfer mail messages to one or more SMTP servers,

   or report its failure to do so.



   The means by which a mail message is presented to an SMTP client, and

   how that client determines the domain name(s) to which mail messages

   are to be transferred is a local matter, and is not addressed by this

   document.  In some cases, the domain name(s) transferred to, or

   determined by, an SMTP client will identify the final destination(s)

   of the mail message.  In other cases, common with SMTP clients

   associated with implementations of the POP [3, 26] or IMAP [6]

   protocols, or when the SMTP client is inside an isolated transport

   service environment, the domain name determined will identify an

   intermediate destination through which all mail messages are to be

   relayed.  SMTP clients that transfer all traffic, regardless of the

   target domain names associated with the individual messages, or that

   do not maintain queues for retrying message transmissions that

   initially cannot be completed, may otherwise conform to this

   specification but are not considered fully-capable.  Fully-capable

   SMTP implementations, including the relays used by these less capable









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   ones, and their destinations, are expected to support all of the

   queuing, retrying, and alternate address functions discussed in this

   specification.



   The means by which an SMTP client, once it has determined a target

   domain name, determines the identity of an SMTP server to which a

   copy of a message is to be transferred, and then performs that

   transfer, is covered by this document.  To effect a mail transfer to

   an SMTP server, an SMTP client establishes a two-way transmission

   channel to that SMTP server.  An SMTP client determines the address

   of an appropriate host running an SMTP server by resolving a

   destination domain name to either an intermediate Mail eXchanger host

   or a final target host.



   An SMTP server may be either the ultimate destination or an

   intermediate "relay" (that is, it may assume the role of an SMTP

   client after receiving the message) or "gateway" (that is, it may

   transport the message further using some protocol other than SMTP).

   SMTP commands are generated by the SMTP client and sent to the SMTP

   server.  SMTP replies are sent from the SMTP server to the SMTP

   client in response to the commands.



   In other words, message transfer can occur in a single connection

   between the original SMTP-sender and the final SMTP-recipient, or can

   occur in a series of hops through intermediary systems.  In either

   case, a formal handoff of responsibility for the message occurs: the

   protocol requires that a server accept responsibility for either

   delivering a message or properly reporting the failure to do so.



   Once the transmission channel is established and initial handshaking

   completed, the SMTP client normally initiates a mail transaction.

   Such a transaction consists of a series of commands to specify the

   originator and destination of the mail and transmission of the

   message content (including any headers or other structure) itself.

   When the same message is sent to multiple recipients, this protocol

   encourages the transmission of only one copy of the data for all

   recipients at the same destination (or intermediate relay) host.



   The server responds to each command with a reply; replies may

   indicate that the command was accepted, that additional commands are

   expected, or that a temporary or permanent error condition exists.

   Commands specifying the sender or recipients may include server-

   permitted SMTP service extension requests as discussed in section

   2.2.  The dialog is purposely lock-step, one-at-a-time, although this

   can be modified by mutually-agreed extension requests such as command

   pipelining [13].











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   Once a given mail message has been transmitted, the client may either

   request that the connection be shut down or may initiate other mail

   transactions.  In addition, an SMTP client may use a connection to an

   SMTP server for ancillary services such as verification of email

   addresses or retrieval of mailing list subscriber addresses.



   As suggested above, this protocol provides mechanisms for the

   transmission of mail.  This transmission normally occurs directly

   from the sending user's host to the receiving user's host when the

   two hosts are connected to the same transport service.  When they are

   not connected to the same transport service, transmission occurs via

   one or more relay SMTP servers.  An intermediate host that acts as

   either an SMTP relay or as a gateway into some other transmission

   environment is usually selected through the use of the domain name

   service (DNS) Mail eXchanger mechanism.



   Usually, intermediate hosts are determined via the DNS MX record, not

   by explicit "source" routing (see section 5 and appendices C and

   F.2).



2.2 The Extension Model



2.2.1 Background



   In an effort that started in 1990, approximately a decade after RFC

   821 was completed, the protocol was modified with a "service

   extensions" model that permits the client and server to agree to

   utilize shared functionality beyond the original SMTP requirements.

   The SMTP extension mechanism defines a means whereby an extended SMTP

   client and server may recognize each other, and the server can inform

   the client as to the service extensions that it supports.



   Contemporary SMTP implementations MUST support the basic extension

   mechanisms.  For instance, servers MUST support the EHLO command even

   if they do not implement any specific extensions and clients SHOULD

   preferentially utilize EHLO rather than HELO.  (However, for

   compatibility with older conforming implementations, SMTP clients and

   servers MUST support the original HELO mechanisms as a fallback.)

   Unless the different characteristics of HELO must be identified for

   interoperability purposes, this document discusses only EHLO.



   SMTP is widely deployed and high-quality implementations have proven

   to be very robust.  However, the Internet community now considers

   some services to be important that were not anticipated when the

   protocol was first designed.  If support for those services is to be

   added, it must be done in a way that permits older implementations to

   continue working acceptably.  The extension framework consists of:









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   -  The SMTP command EHLO, superseding the earlier HELO,



   -  a registry of SMTP service extensions,



   -  additional parameters to the SMTP MAIL and RCPT commands, and



   -  optional replacements for commands defined in this protocol, such

      as for DATA in non-ASCII transmissions [33].



   SMTP's strength comes primarily from its simplicity.  Experience with

   many protocols has shown that protocols with few options tend towards

   ubiquity, whereas protocols with many options tend towards obscurity.



   Each and every extension, regardless of its benefits, must be

   carefully scrutinized with respect to its implementation, deployment,

   and interoperability costs.  In many cases, the cost of extending the

   SMTP service will likely outweigh the benefit.



2.2.2 Definition and Registration of Extensions



   The IANA maintains a registry of SMTP service extensions.  A

   corresponding EHLO keyword value is associated with each extension.

   Each service extension registered with the IANA must be defined in a

   formal standards-track or IESG-approved experimental protocol

   document.  The definition must include:



   -  the textual name of the SMTP service extension;



   -  the EHLO keyword value associated with the extension;



   -  the syntax and possible values of parameters associated with the

      EHLO keyword value;



   -  any additional SMTP verbs associated with the extension

      (additional verbs will usually be, but are not required to be, the

      same as the EHLO keyword value);



   -  any new parameters the extension associates with the MAIL or RCPT

      verbs;



   -  a description of how support for the extension affects the

      behavior of a server and client SMTP; and,



   -  the increment by which the extension is increasing the maximum

      length of the commands MAIL and/or RCPT, over that specified in

      this standard.











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   In addition, any EHLO keyword value starting with an upper or lower

   case "X" refers to a local SMTP service extension used exclusively

   through bilateral agreement.  Keywords beginning with "X" MUST NOT be

   used in a registered service extension.  Conversely, keyword values

   presented in the EHLO response that do not begin with "X" MUST

   correspond to a standard, standards-track, or IESG-approved

   experimental SMTP service extension registered with IANA.  A

   conforming server MUST NOT offer non-"X"-prefixed keyword values that

   are not described in a registered extension.



   Additional verbs and parameter names are bound by the same rules as

   EHLO keywords; specifically, verbs beginning with "X" are local

   extensions that may not be registered or standardized.  Conversely,

   verbs not beginning with "X" must always be registered.



2.3 Terminology



   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

   document are to be interpreted as described below.



   1. MUST   This word, or the terms "REQUIRED" or "SHALL", mean that

      the definition is an absolute requirement of the specification.



   2. MUST NOT   This phrase, or the phrase "SHALL NOT", mean that the

      definition is an absolute prohibition of the specification.



   3. SHOULD   This word, or the adjective "RECOMMENDED", mean that

      there may exist valid reasons in particular circumstances to

      ignore a particular item, but the full implications must be

      understood and carefully weighed before choosing a different

      course.



   4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean

      that there may exist valid reasons in particular circumstances

      when the particular behavior is acceptable or even useful, but the

      full implications should be understood and the case carefully

      weighed before implementing any behavior described with this

      label.



   5. MAY   This word, or the adjective "OPTIONAL", mean that an item is

      truly optional.  One vendor may choose to include the item because

      a particular marketplace requires it or because the vendor feels

      that it enhances the product while another vendor may omit the

      same item.  An implementation which does not include a particular

      option MUST be prepared to interoperate with another

      implementation which does include the option, though perhaps with

      reduced functionality.  In the same vein an implementation which







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      does include a particular option MUST be prepared to interoperate

      with another implementation which does not include the option

      (except, of course, for the feature the option provides.)



2.3.1 Mail Objects



   SMTP transports a mail object.  A mail object contains an envelope

   and content.



   The SMTP envelope is sent as a series of SMTP protocol units

   (described in section 3).  It consists of an originator address (to

   which error reports should be directed); one or more recipient

   addresses; and optional protocol extension material.  Historically,

   variations on the recipient address specification command (RCPT TO)

   could be used to specify alternate delivery modes, such as immediate

   display; those variations have now been deprecated (see appendix F,

   section F.6).



   The SMTP content is sent in the SMTP DATA protocol unit and has two

   parts:  the headers and the body.  If the content conforms to other

   contemporary standards, the headers form a collection of field/value

   pairs structured as in the message format specification [32]; the

   body, if structured, is defined according to MIME [12].  The content

   is textual in nature, expressed using the US-ASCII repertoire [1].

   Although SMTP extensions (such as "8BITMIME" [20]) may relax this

   restriction for the content body, the content headers are always

   encoded using the US-ASCII repertoire.  A MIME extension [23] defines

   an algorithm for representing header values outside the US-ASCII

   repertoire, while still encoding them using the US-ASCII repertoire.



2.3.2 Senders and Receivers



   In RFC 821, the two hosts participating in an SMTP transaction were

   described as the "SMTP-sender" and "SMTP-receiver".  This document

   has been changed to reflect current industry terminology and hence

   refers to them as the "SMTP client" (or sometimes just "the client")

   and "SMTP server" (or just "the server"), respectively.  Since a

   given host may act both as server and client in a relay situation,

   "receiver" and "sender" terminology is still used where needed for

   clarity.



2.3.3 Mail Agents and Message Stores



   Additional mail system terminology became common after RFC 821 was

   published and, where convenient, is used in this specification.  In

   particular, SMTP servers and clients provide a mail transport service

   and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User

   Agents" (MUAs or UAs) are normally thought of as the sources and







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   targets of mail.  At the source, an MUA might collect mail to be

   transmitted from a user and hand it off to an MTA; the final

   ("delivery") MTA would be thought of as handing the mail off to an

   MUA (or at least transferring responsibility to it, e.g., by

   depositing the message in a "message store").  However, while these

   terms are used with at least the appearance of great precision in

   other environments, the implied boundaries between MUAs and MTAs

   often do not accurately match common, and conforming, practices with

   Internet mail.  Hence, the reader should be cautious about inferring

   the strong relationships and responsibilities that might be implied

   if these terms were used elsewhere.



2.3.4 Host



   For the purposes of this specification, a host is a computer system

   attached to the Internet (or, in some cases, to a private TCP/IP

   network) and supporting the SMTP protocol.  Hosts are known by names

   (see "domain"); identifying them by numerical address is discouraged.



2.3.5 Domain



   A domain (or domain name) consists of one or more dot-separated

   components.  These components ("labels" in DNS terminology [22]) are

   restricted for SMTP purposes to consist of a sequence of letters,

   digits, and hyphens drawn from the ASCII character set [1].  Domain

   names are used as names of hosts and of other entities in the domain

   name hierarchy.  For example, a domain may refer to an alias (label

   of a CNAME RR) or the label of Mail eXchanger records to be used to

   deliver mail instead of representing a host name.  See [22] and

   section 5 of this specification.



   The domain name, as described in this document and in [22], is the

   entire, fully-qualified name (often referred to as an "FQDN").  A

   domain name that is not in FQDN form is no more than a local alias.

   Local aliases MUST NOT appear in any SMTP transaction.



2.3.6 Buffer and State Table



   SMTP sessions are stateful, with both parties carefully maintaining a

   common view of the current state.  In this document we model this

   state by a virtual "buffer" and a "state table" on the server which

   may be used by the client to, for example, "clear the buffer" or

   "reset the state table," causing the information in the buffer to be

   discarded and the state to be returned to some previous state.















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2.3.7 Lines



   SMTP commands and, unless altered by a service extension, message

   data, are transmitted in "lines".  Lines consist of zero or more data

   characters terminated by the sequence ASCII character "CR" (hex value

   0D) followed immediately by ASCII character "LF" (hex value 0A).

   This termination sequence is denoted as <CRLF> in this document.

   Conforming implementations MUST NOT recognize or generate any other

   character or character sequence as a line terminator.  Limits MAY be

   imposed on line lengths by servers (see section 4.5.3).



   In addition, the appearance of "bare" "CR" or "LF" characters in text

   (i.e., either without the other) has a long history of causing

   problems in mail implementations and applications that use the mail

   system as a tool.  SMTP client implementations MUST NOT transmit

   these characters except when they are intended as line terminators

   and then MUST, as indicated above, transmit them only as a <CRLF>

   sequence.



2.3.8 Originator, Delivery, Relay, and Gateway Systems



   This specification makes a distinction among four types of SMTP

   systems, based on the role those systems play in transmitting

   electronic mail.  An "originating" system (sometimes called an SMTP

   originator) introduces mail into the Internet or, more generally,

   into a transport service environment.  A "delivery" SMTP system is

   one that receives mail from a transport service environment and

   passes it to a mail user agent or deposits it in a message store

   which a mail user agent is expected to subsequently access.  A

   "relay" SMTP system (usually referred to just as a "relay") receives

   mail from an SMTP client and transmits it, without modification to

   the message data other than adding trace information, to another SMTP

   server for further relaying or for delivery.



   A "gateway" SMTP system (usually referred to just as a "gateway")

   receives mail from a client system in one transport environment and

   transmits it to a server system in another transport environment.

   Differences in protocols or message semantics between the transport

   environments on either side of a gateway may require that the gateway

   system perform transformations to the message that are not permitted

   to SMTP relay systems.  For the purposes of this specification,

   firewalls that rewrite addresses should be considered as gateways,

   even if SMTP is used on both sides of them (see [11]).

















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2.3.9 Message Content and Mail Data



   The terms "message content" and "mail data" are used interchangeably

   in this document to describe the material transmitted after the DATA

   command is accepted and before the end of data indication is

   transmitted.  Message content includes message headers and the

   possibly-structured message body.  The MIME specification [12]

   provides the standard mechanisms for structured message bodies.



2.3.10 Mailbox and Address



   As used in this specification, an "address" is a character string

   that identifies a user to whom mail will be sent or a location into

   which mail will be deposited.  The term "mailbox" refers to that

   depository.  The two terms are typically used interchangeably unless

   the distinction between the location in which mail is placed (the

   mailbox) and a reference to it (the address) is important.  An

   address normally consists of user and domain specifications.  The

   standard mailbox naming convention is defined to be "local-

   part@domain": contemporary usage permits a much broader set of

   applications than simple "user names".  Consequently, and due to a

   long history of problems when intermediate hosts have attempted to

   optimize transport by modifying them, the local-part MUST be

   interpreted and assigned semantics only by the host specified in the

   domain part of the address.



2.3.11 Reply



   An SMTP reply is an acknowledgment (positive or negative) sent from

   receiver to sender via the transmission channel in response to a

   command.  The general form of a reply is a numeric completion code

   (indicating failure or success) usually followed by a text string.

   The codes are for use by programs and the text is usually intended

   for human users.  Recent work [34] has specified further structuring

   of the reply strings, including the use of supplemental and more

   specific completion codes.



2.4 General Syntax Principles and Transaction Model



   SMTP commands and replies have a rigid syntax.  All commands begin

   with a command verb.  All Replies begin with a three digit numeric

   code.  In some commands and replies, arguments MUST follow the verb

   or reply code.  Some commands do not accept arguments (after the

   verb), and some reply codes are followed, sometimes optionally, by

   free form text.  In both cases, where text appears, it is separated

   from the verb or reply code by a space character.  Complete

   definitions of commands and replies appear in section 4.









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   Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command

   and extension name keywords) are not case sensitive, with the sole

   exception in this specification of a mailbox local-part (SMTP

   Extensions may explicitly specify case-sensitive elements).  That is,

   a command verb, an argument value other than a mailbox local-part,

   and free form text MAY be encoded in upper case, lower case, or any

   mixture of upper and lower case with no impact on its meaning.  This

   is NOT true of a mailbox local-part.  The local-part of a mailbox

   MUST BE treated as case sensitive.  Therefore, SMTP implementations

   MUST take care to preserve the case of mailbox local-parts.  Mailbox

   domains are not case sensitive.  In particular, for some hosts the

   user "smith" is different from the user "Smith".  However, exploiting

   the case sensitivity of mailbox local-parts impedes interoperability

   and is discouraged.



   A few SMTP servers, in violation of this specification (and RFC 821)

   require that command verbs be encoded by clients in upper case.

   Implementations MAY wish to employ this encoding to accommodate those

   servers.



   The argument field consists of a variable length character string

   ending with the end of the line, i.e., with the character sequence

   <CRLF>.  The receiver will take no action until this sequence is

   received.



   The syntax for each command is shown with the discussion of that

   command.  Common elements and parameters are shown in section 4.1.2.



   Commands and replies are composed of characters from the ASCII

   character set [1].  When the transport service provides an 8-bit byte

   (octet) transmission channel, each 7-bit character is transmitted

   right justified in an octet with the high order bit cleared to zero.

   More specifically, the unextended SMTP service provides seven bit

   transport only.  An originating SMTP client which has not

   successfully negotiated an appropriate extension with a particular

   server MUST NOT transmit messages with information in the high-order

   bit of octets.  If such messages are transmitted in violation of this

   rule, receiving SMTP servers MAY clear the high-order bit or reject

   the message as invalid.  In general, a relay SMTP SHOULD assume that

   the message content it has received is valid and, assuming that the

   envelope permits doing so, relay it without inspecting that content.

   Of course, if the content is mislabeled and the data path cannot

   accept the actual content, this may result in ultimate delivery of a

   severely garbled message to the recipient.  Delivery SMTP systems MAY

   reject ("bounce") such messages rather than deliver them.  No sending

   SMTP system is permitted to send envelope commands in any character











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   set other than US-ASCII; receiving systems SHOULD reject such

   commands, normally using "500 syntax error - invalid character"

   replies.



   Eight-bit message content transmission MAY be requested of the server

   by a client using extended SMTP facilities, notably the "8BITMIME"

   extension [20].  8BITMIME SHOULD be supported by SMTP servers.

   However, it MUST not be construed as authorization to transmit

   unrestricted eight bit material.  8BITMIME MUST NOT be requested by

   senders for material with the high bit on that is not in MIME format

   with an appropriate content-transfer encoding; servers MAY reject

   such messages.



   The metalinguistic notation used in this document corresponds to the

   "Augmented BNF" used in other Internet mail system documents.  The

   reader who is not familiar with that syntax should consult the ABNF

   specification [8].  Metalanguage terms used in running text are

   surrounded by pointed brackets (e.g., <CRLF>) for clarity.



3. The SMTP Procedures: An Overview



   This section contains descriptions of the procedures used in SMTP:

   session initiation, the mail transaction, forwarding mail, verifying

   mailbox names and expanding mailing lists, and the opening and

   closing exchanges.  Comments on relaying, a note on mail domains, and

   a discussion of changing roles are included at the end of this

   section.  Several complete scenarios are presented in appendix D.



3.1 Session Initiation



   An SMTP session is initiated when a client opens a connection to a

   server and the server responds with an opening message.



   SMTP server implementations MAY include identification of their

   software and version information in the connection greeting reply

   after the 220 code, a practice that permits more efficient isolation

   and repair of any problems.  Implementations MAY make provision for

   SMTP servers to disable the software and version announcement where

   it causes security concerns.  While some systems also identify their

   contact point for mail problems, this is not a substitute for

   maintaining the required "postmaster" address (see section 4.5.1).



   The SMTP protocol allows a server to formally reject a transaction

   while still allowing the initial connection as follows: a 554

   response MAY be given in the initial connection opening message

   instead of the 220.  A server taking this approach MUST still wait

   for the client to send a QUIT (see section 4.1.1.10) before closing

   the connection and SHOULD respond to any intervening commands with







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   "503 bad sequence of commands".  Since an attempt to make an SMTP

   connection to such a system is probably in error, a server returning

   a 554 response on connection opening SHOULD provide enough

   information in the reply text to facilitate debugging of the sending

   system.



3.2 Client Initiation



   Once the server has sent the welcoming message and the client has

   received it, the client normally sends the EHLO command to the

   server, indicating the client's identity.  In addition to opening the

   session, use of EHLO indicates that the client is able to process

   service extensions and requests that the server provide a list of the

   extensions it supports.  Older SMTP systems which are unable to

   support service extensions and contemporary clients which do not

   require service extensions in the mail session being initiated, MAY

   use HELO instead of EHLO.  Servers MUST NOT return the extended

   EHLO-style response to a HELO command.  For a particular connection

   attempt, if the server returns a "command not recognized" response to

   EHLO, the client SHOULD be able to fall back and send HELO.



   In the EHLO command the host sending the command identifies itself;

   the command may be interpreted as saying "Hello, I am <domain>" (and,

   in the case of EHLO, "and I support service extension requests").



3.3 Mail Transactions



   There are three steps to SMTP mail transactions.  The transaction

   starts with a MAIL command which gives the sender identification.

   (In general, the MAIL command may be sent only when no mail

   transaction is in progress; see section 4.1.4.)  A series of one or

   more RCPT commands follows giving the receiver information.  Then a

   DATA command initiates transfer of the mail data and is terminated by

   the "end of mail" data indicator, which also confirms the

   transaction.



   The first step in the procedure is the MAIL command.



      MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>



   This command tells the SMTP-receiver that a new mail transaction is

   starting and to reset all its state tables and buffers, including any

   recipients or mail data.  The <reverse-path> portion of the first or

   only argument contains the source mailbox (between "<" and ">"

   brackets), which can be used to report errors (see section 4.2 for a

   discussion of error reporting).  If accepted, the SMTP server returns

   a 250 OK reply.  If the mailbox specification is not acceptable for

   some reason, the server MUST return a reply indicating whether the







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   failure is permanent (i.e., will occur again if the client tries to

   send the same address again) or temporary (i.e., the address might be

   accepted if the client tries again later).  Despite the apparent

   scope of this requirement, there are circumstances in which the

   acceptability of the reverse-path may not be determined until one or

   more forward-paths (in RCPT commands) can be examined.  In those

   cases, the server MAY reasonably accept the reverse-path (with a 250

   reply) and then report problems after the forward-paths are received

   and examined.  Normally, failures produce 550 or 553 replies.



   Historically, the <reverse-path> can contain more than just a

   mailbox, however, contemporary systems SHOULD NOT use source routing

   (see appendix C).



   The optional <mail-parameters> are associated with negotiated SMTP

   service extensions (see section 2.2).



   The second step in the procedure is the RCPT command.



      RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>



   The first or only argument to this command includes a forward-path

   (normally a mailbox and domain, always surrounded by "<" and ">"

   brackets) identifying one recipient.  If accepted, the SMTP server

   returns a 250 OK reply and stores the forward-path.  If the recipient

   is known not to be a deliverable address, the SMTP server returns a

   550 reply, typically with a string such as "no such user - " and the

   mailbox name (other circumstances and reply codes are possible).

   This step of the procedure can be repeated any number of times.



   The <forward-path> can contain more than just a mailbox.

   Historically, the <forward-path> can be a source routing list of

   hosts and the destination mailbox, however, contemporary SMTP clients

   SHOULD NOT utilize source routes (see appendix C).  Servers MUST be

   prepared to encounter a list of source routes in the forward path,

   but SHOULD ignore the routes or MAY decline to support the relaying

   they imply.  Similarly, servers MAY decline to accept mail that is

   destined for other hosts or systems.  These restrictions make a

   server useless as a relay for clients that do not support full SMTP

   functionality.  Consequently, restricted-capability clients MUST NOT

   assume that any SMTP server on the Internet can be used as their mail

   processing (relaying) site.  If a RCPT command appears without a

   previous MAIL command, the server MUST return a 503 "Bad sequence of

   commands" response.  The optional <rcpt-parameters> are associated

   with negotiated SMTP service extensions (see section 2.2).



   The third step in the procedure is the DATA command (or some

   alternative specified in a service extension).







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      DATA <CRLF>



   If accepted, the SMTP server returns a 354 Intermediate reply and

   considers all succeeding lines up to but not including the end of

   mail data indicator to be the message text.  When the end of text is

   successfully received and stored the SMTP-receiver sends a 250 OK

   reply.



   Since the mail data is sent on the transmission channel, the end of

   mail data must be indicated so that the command and reply dialog can

   be resumed.  SMTP indicates the end of the mail data by sending a

   line containing only a "." (period or full stop).  A transparency

   procedure is used to prevent this from interfering with the user's

   text (see section 4.5.2).



   The end of mail data indicator also confirms the mail transaction and

   tells the SMTP server to now process the stored recipients and mail

   data.  If accepted, the SMTP server returns a 250 OK reply.  The DATA

   command can fail at only two points in the protocol exchange:



   -  If there was no MAIL, or no RCPT, command, or all such commands

      were rejected, the server MAY return a "command out of sequence"

      (503) or "no valid recipients" (554) reply in response to the DATA

      command.  If one of those replies (or any other 5yz reply) is

      received, the client MUST NOT send the message data; more

      generally, message data MUST NOT be sent unless a 354 reply is

      received.



   -  If the verb is initially accepted and the 354 reply issued, the

      DATA command should fail only if the mail transaction was

      incomplete (for example, no recipients), or if resources were

      unavailable (including, of course, the server unexpectedly

      becoming unavailable), or if the server determines that the

      message should be rejected for policy or other reasons.



   However, in practice, some servers do not perform recipient

   verification until after the message text is received.  These servers

   SHOULD treat a failure for one or more recipients as a "subsequent

   failure" and return a mail message as discussed in section 6.  Using

   a "550 mailbox not found" (or equivalent) reply code after the data

   are accepted makes it difficult or impossible for the client to

   determine which recipients failed.



   When RFC 822 format [7, 32] is being used, the mail data include the

   memo header items such as Date, Subject, To, Cc, From.  Server SMTP

   systems SHOULD NOT reject messages based on perceived defects in the

   RFC 822 or MIME [12] message header or message body.  In particular,









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   they MUST NOT reject messages in which the numbers of Resent-fields

   do not match or Resent-to appears without Resent-from and/or Resent-

   date.



   Mail transaction commands MUST be used in the order discussed above.



3.4 Forwarding for Address Correction or Updating



   Forwarding support is most often required to consolidate and simplify

   addresses within, or relative to, some enterprise and less frequently

   to establish addresses to link a person's prior address with current

   one.  Silent forwarding of messages (without server notification to

   the sender), for security or non-disclosure purposes, is common in

   the contemporary Internet.



   In both the enterprise and the "new address" cases, information

   hiding (and sometimes security) considerations argue against exposure

   of the "final" address through the SMTP protocol as a side-effect of

   the forwarding activity.  This may be especially important when the

   final address may not even be reachable by the sender.  Consequently,

   the "forwarding" mechanisms described in section 3.2 of RFC 821, and

   especially the 251 (corrected destination) and 551 reply codes from

   RCPT must be evaluated carefully by implementers and, when they are

   available, by those configuring systems.



   In particular:



   *  Servers MAY forward messages when they are aware of an address

      change.  When they do so, they MAY either provide address-updating

      information with a 251 code, or may forward "silently" and return

      a 250 code.  But, if a 251 code is used, they MUST NOT assume that

      the client will actually update address information or even return

      that information to the user.



   Alternately,



   *  Servers MAY reject or bounce messages when they are not

      deliverable when addressed.  When they do so, they MAY either

      provide address-updating information with a 551 code, or may

      reject the message as undeliverable with a 550 code and no

      address-specific information.  But, if a 551 code is used, they

      MUST NOT assume that the client will actually update address

      information or even return that information to the user.



   SMTP server implementations that support the 251 and/or 551 reply

   codes are strongly encouraged to provide configuration mechanisms so

   that sites which conclude that they would undesirably disclose

   information can disable or restrict their use.







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3.5 Commands for Debugging Addresses



3.5.1 Overview



   SMTP provides commands to verify a user name or obtain the content of

   a mailing list.  This is done with the VRFY and EXPN commands, which

   have character string arguments.  Implementations SHOULD support VRFY

   and EXPN (however, see section 3.5.2 and 7.3).



   For the VRFY command, the string is a user name or a user name and

   domain (see below).  If a normal (i.e., 250) response is returned,

   the response MAY include the full name of the user and MUST include

   the mailbox of the user.  It MUST be in either of the following

   forms:



      User Name <local-part@domain>

      local-part@domain



   When a name that is the argument to VRFY could identify more than one

   mailbox, the server MAY either note the ambiguity or identify the

   alternatives.  In other words, any of the following are legitimate

   response to VRFY:



      553 User ambiguous



   or



      553- Ambiguous;  Possibilities are

      553-Joe Smith <jsmith@foo.com>

      553-Harry Smith <hsmith@foo.com>

      553 Melvin Smith <dweep@foo.com>



   or



      553-Ambiguous;  Possibilities

      553- <jsmith@foo.com>

      553- <hsmith@foo.com>

      553 <dweep@foo.com>



   Under normal circumstances, a client receiving a 553 reply would be

   expected to expose the result to the user.  Use of exactly the forms

   given, and the "user ambiguous" or "ambiguous" keywords, possibly

   supplemented by extended reply codes such as those described in [34],

   will facilitate automated translation into other languages as needed.

   Of course, a client that was highly automated or that was operating

   in another language than English, might choose to try to translate

   the response, to return some other indication to the user than the









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   literal text of the reply, or to take some automated action such as

   consulting a directory service for additional information before

   reporting to the user.



   For the EXPN command, the string identifies a mailing list, and the

   successful (i.e., 250) multiline response MAY include the full name

   of the users and MUST give the mailboxes on the mailing list.



   In some hosts the distinction between a mailing list and an alias for

   a single mailbox is a bit fuzzy, since a common data structure may

   hold both types of entries, and it is possible to have mailing lists

   containing only one mailbox.  If a request is made to apply VRFY to a

   mailing list, a positive response MAY be given if a message so

   addressed would be delivered to everyone on the list, otherwise an

   error SHOULD be reported (e.g., "550 That is a mailing list, not a

   user" or "252 Unable to verify members of mailing list").  If a

   request is made to expand a user name, the server MAY return a

   positive response consisting of a list containing one name, or an

   error MAY be reported (e.g., "550 That is a user name, not a mailing

   list").



   In the case of a successful multiline reply (normal for EXPN) exactly

   one mailbox is to be specified on each line of the reply.  The case

   of an ambiguous request is discussed above.



   "User name" is a fuzzy term and has been used deliberately.  An

   implementation of the VRFY or EXPN commands MUST include at least

   recognition of local mailboxes as "user names".  However, since

   current Internet practice often results in a single host handling

   mail for multiple domains, hosts, especially hosts that provide this

   functionality, SHOULD accept the "local-part@domain" form as a "user

   name"; hosts MAY also choose to recognize other strings as "user

   names".



   The case of expanding a mailbox list requires a multiline reply, such

   as:



      C: EXPN Example-People

      S: 250-Jon Postel <Postel@isi.edu>

      S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>

      S: 250 Sam Q. Smith <SQSmith@specific.generic.com>



   or



      C: EXPN Executive-Washroom-List

      S: 550 Access Denied to You.











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   The character string arguments of the VRFY and EXPN commands cannot

   be further restricted due to the variety of implementations of the

   user name and mailbox list concepts.  On some systems it may be

   appropriate for the argument of the EXPN command to be a file name

   for a file containing a mailing list, but again there are a variety

   of file naming conventions in the Internet.  Similarly, historical

   variations in what is returned by these commands are such that the

   response SHOULD be interpreted very carefully, if at all, and SHOULD

   generally only be used for diagnostic purposes.



3.5.2 VRFY Normal Response



   When normal (2yz or 551) responses are returned from a VRFY or EXPN

   request, the reply normally includes the mailbox name, i.e.,

   "<local-part@domain>", where "domain" is a fully qualified domain

   name, MUST appear in the syntax.  In circumstances exceptional enough

   to justify violating the intent of this specification, free-form text

   MAY be returned.  In order to facilitate parsing by both computers

   and people, addresses SHOULD appear in pointed brackets.  When

   addresses, rather than free-form debugging information, are returned,

   EXPN and VRFY MUST return only valid domain addresses that are usable

   in SMTP RCPT commands.  Consequently, if an address implies delivery

   to a program or other system, the mailbox name used to reach that

   target MUST be given.  Paths (explicit source routes) MUST NOT be

   returned by VRFY or EXPN.



   Server implementations SHOULD support both VRFY and EXPN.  For

   security reasons, implementations MAY provide local installations a

   way to disable either or both of these commands through configuration

   options or the equivalent.  When these commands are supported, they

   are not required to work across relays when relaying is supported.

   Since they were both optional in RFC 821, they MUST be listed as

   service extensions in an EHLO response, if they are supported.



3.5.3 Meaning of VRFY or EXPN Success Response



   A server MUST NOT return a 250 code in response to a VRFY or EXPN

   command unless it has actually verified the address.  In particular,

   a server MUST NOT return 250 if all it has done is to verify that the

   syntax given is valid.  In that case, 502 (Command not implemented)

   or 500 (Syntax error, command unrecognized) SHOULD be returned.  As

   stated elsewhere, implementation (in the sense of actually validating

   addresses and returning information) of VRFY and EXPN are strongly

   recommended.  Hence, implementations that return 500 or 502 for VRFY

   are not in full compliance with this specification.













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   There may be circumstances where an address appears to be valid but

   cannot reasonably be verified in real time, particularly when a

   server is acting as a mail exchanger for another server or domain.

   "Apparent validity" in this case would normally involve at least

   syntax checking and might involve verification that any domains

   specified were ones to which the host expected to be able to relay

   mail.  In these situations, reply code 252 SHOULD be returned.  These

   cases parallel the discussion of RCPT verification discussed in

   section 2.1.  Similarly, the discussion in section 3.4 applies to the

   use of reply codes 251 and 551 with VRFY (and EXPN) to indicate

   addresses that are recognized but that would be forwarded or bounced

   were mail received for them.  Implementations generally SHOULD be

   more aggressive about address verification in the case of VRFY than

   in the case of RCPT, even if it takes a little longer to do so.



3.5.4 Semantics and Applications of EXPN



   EXPN is often very useful in debugging and understanding problems

   with mailing lists and multiple-target-address aliases.  Some systems

   have attempted to use source expansion of mailing lists as a means of

   eliminating duplicates.  The propagation of aliasing systems with

   mail on the Internet, for hosts (typically with MX and CNAME DNS

   records), for mailboxes (various types of local host aliases), and in

   various proxying arrangements, has made it nearly impossible for

   these strategies to work consistently, and mail systems SHOULD NOT

   attempt them.



3.6 Domains



   Only resolvable, fully-qualified, domain names (FQDNs) are permitted

   when domain names are used in SMTP.  In other words, names that can

   be resolved to MX RRs or A RRs (as discussed in section 5) are

   permitted, as are CNAME RRs whose targets can be resolved, in turn,

   to MX or A RRs.  Local nicknames or unqualified names MUST NOT be

   used.  There are two exceptions to the rule requiring FQDNs:



   -  The domain name given in the EHLO command MUST BE either a primary

      host name (a domain name that resolves to an A RR) or, if the host

      has no name, an address literal as described in section 4.1.1.1.



   -  The reserved mailbox name "postmaster" may be used in a RCPT

      command without domain qualification (see section 4.1.1.3) and

      MUST be accepted if so used.

















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3.7 Relaying



   In general, the availability of Mail eXchanger records in the domain

   name system [22, 27] makes the use of explicit source routes in the

   Internet mail system unnecessary.  Many historical problems with

   their interpretation have made their use undesirable.  SMTP clients

   SHOULD NOT generate explicit source routes except under unusual

   circumstances.  SMTP servers MAY decline to act as mail relays or to

   accept addresses that specify source routes.  When route information

   is encountered, SMTP servers are also permitted to ignore the route

   information and simply send to the final destination specified as the

   last element in the route and SHOULD do so.  There has been an

   invalid practice of using names that do not appear in the DNS as

   destination names, with the senders counting on the intermediate

   hosts specified in source routing to resolve any problems.  If source

   routes are stripped, this practice will cause failures.  This is one

   of several reasons why SMTP clients MUST NOT generate invalid source

   routes or depend on serial resolution of names.



   When source routes are not used, the process described in RFC 821 for

   constructing a reverse-path from the forward-path is not applicable

   and the reverse-path at the time of delivery will simply be the

   address that appeared in the MAIL command.



   A relay SMTP server is usually the target of a DNS MX record that

   designates it, rather than the final delivery system.  The relay

   server may accept or reject the task of relaying the mail in the same

   way it accepts or rejects mail for a local user.  If it accepts the

   task, it then becomes an SMTP client, establishes a transmission

   channel to the next SMTP server specified in the DNS (according to

   the rules in section 5), and sends it the mail.  If it declines to

   relay mail to a particular address for policy reasons, a 550 response

   SHOULD be returned.



   Many mail-sending clients exist, especially in conjunction with

   facilities that receive mail via POP3 or IMAP, that have limited

   capability to support some of the requirements of this specification,

   such as the ability to queue messages for subsequent delivery

   attempts.  For these clients, it is common practice to make private

   arrangements to send all messages to a single server for processing

   and subsequent distribution.  SMTP, as specified here, is not ideally

   suited for this role, and work is underway on standardized mail

   submission protocols that might eventually supercede the current

   practices.  In any event, because these arrangements are private and

   fall outside the scope of this specification, they are not described

   here.











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   It is important to note that MX records can point to SMTP servers

   which act as gateways into other environments, not just SMTP relays

   and final delivery systems; see sections 3.8 and 5.



   If an SMTP server has accepted the task of relaying the mail and

   later finds that the destination is incorrect or that the mail cannot

   be delivered for some other reason, then it MUST construct an

   "undeliverable mail" notification message and send it to the

   originator of the undeliverable mail (as indicated by the reverse-

   path).  Formats specified for non-delivery reports by other standards

   (see, for example, [24, 25]) SHOULD be used if possible.



   This notification message must be from the SMTP server at the relay

   host or the host that first determines that delivery cannot be

   accomplished.  Of course, SMTP servers MUST NOT send notification

   messages about problems transporting notification messages.  One way

   to prevent loops in error reporting is to specify a null reverse-path

   in the MAIL command of a notification message.  When such a message

   is transmitted the reverse-path MUST be set to null (see section

   4.5.5 for additional discussion).  A MAIL command with a null

   reverse-path appears as follows:



      MAIL FROM:<>



   As discussed in section 2.4.1, a relay SMTP has no need to inspect or

   act upon the headers or body of the message data and MUST NOT do so

   except to add its own "Received:" header (section 4.4) and,

   optionally, to attempt to detect looping in the mail system (see

   section 6.2).



3.8 Mail Gatewaying



   While the relay function discussed above operates within the Internet

   SMTP transport service environment, MX records or various forms of

   explicit routing may require that an intermediate SMTP server perform

   a translation function between one transport service and another.  As

   discussed in section 2.3.8, when such a system is at the boundary

   between two transport service environments, we refer to it as a

   "gateway" or "gateway SMTP".



   Gatewaying mail between different mail environments, such as

   different mail formats and protocols, is complex and does not easily

   yield to standardization.  However, some general requirements may be

   given for a gateway between the Internet and another mail

   environment.













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3.8.1 Header Fields in Gatewaying



   Header fields MAY be rewritten when necessary as messages are

   gatewayed across mail environment boundaries.  This may involve

   inspecting the message body or interpreting the local-part of the

   destination address in spite of the prohibitions in section 2.4.1.



   Other mail systems gatewayed to the Internet often use a subset of

   RFC 822 headers or provide similar functionality with a different

   syntax, but some of these mail systems do not have an equivalent to

   the SMTP envelope.  Therefore, when a message leaves the Internet

   environment, it may be necessary to fold the SMTP envelope

   information into the message header.  A possible solution would be to

   create new header fields to carry the envelope information (e.g.,

   "X-SMTP-MAIL:"  and "X-SMTP-RCPT:"); however, this would require

   changes in mail programs in foreign environments and might risk

   disclosure of private information (see section 7.2).



3.8.2 Received Lines in Gatewaying



   When forwarding a message into or out of the Internet environment, a

   gateway MUST prepend a Received: line, but it MUST NOT alter in any

   way a Received: line that is already in the header.



   "Received:" fields of messages originating from other environments

   may not conform exactly to this specification.  However, the most

   important use of Received: lines is for debugging mail faults, and

   this debugging can be severely hampered by well-meaning gateways that

   try to "fix" a Received: line.  As another consequence of trace

   fields arising in non-SMTP environments, receiving systems MUST NOT

   reject mail based on the format of a trace field and SHOULD be

   extremely robust in the light of unexpected information or formats in

   those fields.



   The gateway SHOULD indicate the environment and protocol in the "via"

   clauses of Received field(s) that it supplies.



3.8.3 Addresses in Gatewaying



   From the Internet side, the gateway SHOULD accept all valid address

   formats in SMTP commands and in RFC 822 headers, and all valid RFC

   822 messages.  Addresses and headers generated by gateways MUST

   conform to applicable Internet standards (including this one and RFC

   822).  Gateways are, of course, subject to the same rules for

   handling source routes as those described for other SMTP systems in

   section 3.3.











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3.8.4 Other Header Fields in Gatewaying



   The gateway MUST ensure that all header fields of a message that it

   forwards into the Internet mail environment meet the requirements for

   Internet mail.  In particular, all addresses in "From:", "To:",

   "Cc:", etc., fields MUST be transformed (if necessary) to satisfy RFC

   822 syntax, MUST reference only fully-qualified domain names, and

   MUST be effective and useful for sending replies.  The translation

   algorithm used to convert mail from the Internet protocols to another

   environment's protocol SHOULD ensure that error messages from the

   foreign mail environment are delivered to the return path from the

   SMTP envelope, not to the sender listed in the "From:" field (or

   other fields) of the RFC 822 message.



3.8.5 Envelopes in Gatewaying



   Similarly, when forwarding a message from another environment into

   the Internet, the gateway SHOULD set the envelope return path in

   accordance with an error message return address, if supplied by the

   foreign environment.  If the foreign environment has no equivalent

   concept, the gateway must select and use a best approximation, with

   the message originator's address as the default of last resort.



3.9 Terminating Sessions and Connections



   An SMTP connection is terminated when the client sends a QUIT

   command.  The server responds with a positive reply code, after which

   it closes the connection.



   An SMTP server MUST NOT intentionally close the connection except:



   -  After receiving a QUIT command and responding with a 221 reply.



   -  After detecting the need to shut down the SMTP service and

      returning a 421 response code.  This response code can be issued

      after the server receives any command or, if necessary,

      asynchronously from command receipt (on the assumption that the

      client will receive it after the next command is issued).



   In particular, a server that closes connections in response to

   commands that are not understood is in violation of this

   specification.  Servers are expected to be tolerant of unknown

   commands, issuing a 500 reply and awaiting further instructions from

   the client.















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   An SMTP server which is forcibly shut down via external means SHOULD

   attempt to send a line containing a 421 response code to the SMTP

   client before exiting.  The SMTP client will normally read the 421

   response code after sending its next command.



   SMTP clients that experience a connection close, reset, or other

   communications failure due to circumstances not under their control

   (in violation of the intent of this specification but sometimes

   unavoidable) SHOULD, to maintain the robustness of the mail system,

   treat the mail transaction as if a 451 response had been received and

   act accordingly.



3.10 Mailing Lists and Aliases



   An SMTP-capable host SHOULD support both the alias and the list

   models of address expansion for multiple delivery.  When a message is

   delivered or forwarded to each address of an expanded list form, the

   return address in the envelope ("MAIL FROM:") MUST be changed to be

   the address of a person or other entity who administers the list.

   However, in this case, the message header [32] MUST be left

   unchanged; in particular, the "From" field of the message header is

   unaffected.



   An important mail facility is a mechanism for multi-destination

   delivery of a single message, by transforming (or "expanding" or

   "exploding") a pseudo-mailbox address into a list of destination

   mailbox addresses.  When a message is sent to such a pseudo-mailbox

   (sometimes called an "exploder"), copies are forwarded or

   redistributed to each mailbox in the expanded list.  Servers SHOULD

   simply utilize the addresses on the list; application of heuristics

   or other matching rules to eliminate some addresses, such as that of

   the originator, is strongly discouraged.  We classify such a pseudo-

   mailbox as an "alias" or a "list", depending upon the expansion

   rules.



3.10.1 Alias



   To expand an alias, the recipient mailer simply replaces the pseudo-

   mailbox address in the envelope with each of the expanded addresses

   in turn; the rest of the envelope and the message body are left

   unchanged.  The message is then delivered or forwarded to each

   expanded address.



3.10.2 List



   A mailing list may be said to operate by "redistribution" rather than

   by "forwarding".  To expand a list, the recipient mailer replaces the

   pseudo-mailbox address in the envelope with all of the expanded







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   addresses.  The return address in the envelope is changed so that all

   error messages generated by the final deliveries will be returned to

   a list administrator, not to the message originator, who generally

   has no control over the contents of the list and will typically find

   error messages annoying.



4. The SMTP Specifications



4.1 SMTP Commands



4.1.1 Command Semantics and Syntax



   The SMTP commands define the mail transfer or the mail system

   function requested by the user.  SMTP commands are character strings

   terminated by <CRLF>.  The commands themselves are alphabetic

   characters terminated by <SP> if parameters follow and <CRLF>

   otherwise.  (In the interest of improved interoperability, SMTP

   receivers are encouraged to tolerate trailing white space before the

   terminating <CRLF>.)  The syntax of the local part of a mailbox must

   conform to receiver site conventions and the syntax specified in

   section 4.1.2.  The SMTP commands are discussed below.  The SMTP

   replies are discussed in section 4.2.



   A mail transaction involves several data objects which are

   communicated as arguments to different commands.  The reverse-path is

   the argument of the MAIL command, the forward-path is the argument of

   the RCPT command, and the mail data is the argument of the DATA

   command.  These arguments or data objects must be transmitted and

   held pending the confirmation communicated by the end of mail data

   indication which finalizes the transaction.  The model for this is

   that distinct buffers are provided to hold the types of data objects,

   that is, there is a reverse-path buffer, a forward-path buffer, and a

   mail data buffer.  Specific commands cause information to be appended

   to a specific buffer, or cause one or more buffers to be cleared.



   Several commands (RSET, DATA, QUIT) are specified as not permitting

   parameters.  In the absence of specific extensions offered by the

   server and accepted by the client, clients MUST NOT send such

   parameters and servers SHOULD reject commands containing them as

   having invalid syntax.



4.1.1.1  Extended HELLO (EHLO) or HELLO (HELO)



   These commands are used to identify the SMTP client to the SMTP

   server.  The argument field contains the fully-qualified domain name

   of the SMTP client if one is available.  In situations in which the

   SMTP client system does not have a meaningful domain name (e.g., when

   its address is dynamically allocated and no reverse mapping record is







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   available), the client SHOULD send an address literal (see section

   4.1.3), optionally followed by information that will help to identify

   the client system.  y The SMTP server identifies itself to the SMTP

   client in the connection greeting reply and in the response to this

   command.



   A client SMTP SHOULD start an SMTP session by issuing the EHLO

   command.  If the SMTP server supports the SMTP service extensions it

   will give a successful response, a failure response, or an error

   response.  If the SMTP server, in violation of this specification,

   does not support any SMTP service extensions it will generate an

   error response.  Older client SMTP systems MAY, as discussed above,

   use HELO (as specified in RFC 821) instead of EHLO, and servers MUST

   support the HELO command and reply properly to it.  In any event, a

   client MUST issue HELO or EHLO before starting a mail transaction.



   These commands, and a "250 OK" reply to one of them, confirm that

   both the SMTP client and the SMTP server are in the initial state,

   that is, there is no transaction in progress and all state tables and

   buffers are cleared.



   Syntax:



      ehlo            = "EHLO" SP Domain CRLF

      helo            = "HELO" SP Domain CRLF



   Normally, the response to EHLO will be a multiline reply.  Each line

   of the response contains a keyword and, optionally, one or more

   parameters.  Following the normal syntax for multiline replies, these

   keyworks follow the code (250) and a hyphen for all but the last

   line, and the code and a space for the last line.  The syntax for a

   positive response, using the ABNF notation and terminal symbols of

   [8], is:



      ehlo-ok-rsp  =    ( "250"    domain [ SP ehlo-greet ] CRLF )

                   / (    "250-"   domain [ SP ehlo-greet ] CRLF

                       *( "250-"   ehlo-line                CRLF )

                          "250"    SP ehlo-line             CRLF  )



      ehlo-greet   = 1*(%d0-9 / %d11-12 / %d14-127)

                   ; string of any characters other than CR or LF



      ehlo-line    = ehlo-keyword *( SP ehlo-param )



      ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")

                   ; additional syntax of ehlo-params depends on

                   ; ehlo-keyword









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      ehlo-param   = 1*(%d33-127)

                   ; any CHAR excluding <SP> and all

                   ; control characters (US-ASCII 0-31 inclusive)



   Although EHLO keywords may be specified in upper, lower, or mixed

   case, they MUST always be recognized and processed in a case-

   insensitive manner.  This is simply an extension of practices

   specified in RFC 821 and section 2.4.1.



4.1.1.2 MAIL (MAIL)



   This command is used to initiate a mail transaction in which the mail

   data is delivered to an SMTP server which may, in turn, deliver it to

   one or more mailboxes or pass it on to another system (possibly using

   SMTP).  The argument field contains a reverse-path and may contain

   optional parameters.  In general, the MAIL command may be sent only

   when no mail transaction is in progress, see section 4.1.4.



   The reverse-path consists of the sender mailbox.  Historically, that

   mailbox might optionally have been preceded by a list of hosts, but

   that behavior is now deprecated (see appendix C).  In some types of

   reporting messages for which a reply is likely to cause a mail loop

   (for example, mail delivery and nondelivery notifications), the

   reverse-path may be null (see section 3.7).



   This command clears the reverse-path buffer, the forward-path buffer,

   and the mail data buffer; and inserts the reverse-path information

   from this command into the reverse-path buffer.



   If service extensions were negotiated, the MAIL command may also

   carry parameters associated with a particular service extension.



   Syntax:



      "MAIL FROM:" ("<>" / Reverse-Path)

                       [SP Mail-parameters] CRLF



4.1.1.3 RECIPIENT (RCPT)



   This command is used to identify an individual recipient of the mail

   data; multiple recipients are specified by multiple use of this

   command.  The argument field contains a forward-path and may contain

   optional parameters.



   The forward-path normally consists of the required destination

   mailbox.  Sending systems SHOULD not generate the optional list of

   hosts known as a source route.  Receiving systems MUST recognize









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   source route syntax but SHOULD strip off the source route

   specification and utilize the domain name associated with the mailbox

   as if the source route had not been provided.



   Similarly, relay hosts SHOULD strip or ignore source routes, and

   names MUST NOT be copied into the reverse-path.  When mail reaches

   its ultimate destination (the forward-path contains only a

   destination mailbox), the SMTP server inserts it into the destination

   mailbox in accordance with its host mail conventions.



   For example, mail received at relay host xyz.com with envelope

   commands



      MAIL FROM:<userx@y.foo.org>

      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>



   will normally be sent directly on to host d.bar.org with envelope

   commands



      MAIL FROM:<userx@y.foo.org>

      RCPT TO:<userc@d.bar.org>



   As provided in appendix C, xyz.com MAY also choose to relay the

   message to hosta.int, using the envelope commands



      MAIL FROM:<userx@y.foo.org>

      RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>



   or to jkl.org, using the envelope commands



      MAIL FROM:<userx@y.foo.org>

      RCPT TO:<@jkl.org:userc@d.bar.org>



   Of course, since hosts are not required to relay mail at all, xyz.com

   may also reject the message entirely when the RCPT command is

   received, using a 550 code (since this is a "policy reason").



   If service extensions were negotiated, the RCPT command may also

   carry parameters associated with a particular service extension

   offered by the server.  The client MUST NOT transmit parameters other

   than those associated with a service extension offered by the server

   in its EHLO response.



Syntax:

   "RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)

                    [SP Rcpt-parameters] CRLF











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4.1.1.4 DATA (DATA)



   The receiver normally sends a 354 response to DATA, and then treats

   the lines (strings ending in <CRLF> sequences, as described in

   section 2.3.7) following the command as mail data from the sender.

   This command causes the mail data to be appended to the mail data

   buffer.  The mail data may contain any of the 128 ASCII character

   codes, although experience has indicated that use of control

   characters other than SP, HT, CR, and LF may cause problems and

   SHOULD be avoided when possible.



   The mail data is terminated by a line containing only a period, that

   is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2).  This

   is the end of mail data indication.  Note that the first <CRLF> of

   this terminating sequence is also the <CRLF> that ends the final line

   of the data (message text) or, if there was no data, ends the DATA

   command itself.  An extra <CRLF> MUST NOT be added, as that would

   cause an empty line to be added to the message.  The only exception

   to this rule would arise if the message body were passed to the

   originating SMTP-sender with a final "line" that did not end in

   <CRLF>; in that case, the originating SMTP system MUST either reject

   the message as invalid or add <CRLF> in order to have the receiving

   SMTP server recognize the "end of data" condition.



   The custom of accepting lines ending only in <LF>, as a concession to

   non-conforming behavior on the part of some UNIX systems, has proven

   to cause more interoperability problems than it solves, and SMTP

   server systems MUST NOT do this, even in the name of improved

   robustness.  In particular, the sequence "<LF>.<LF>" (bare line

   feeds, without carriage returns) MUST NOT be treated as equivalent to

   <CRLF>.<CRLF> as the end of mail data indication.



   Receipt of the end of mail data indication requires the server to

   process the stored mail transaction information.  This processing

   consumes the information in the reverse-path buffer, the forward-path

   buffer, and the mail data buffer, and on the completion of this

   command these buffers are cleared.  If the processing is successful,

   the receiver MUST send an OK reply.  If the processing fails the

   receiver MUST send a failure reply.  The SMTP model does not allow

   for partial failures at this point: either the message is accepted by

   the server for delivery and a positive response is returned or it is

   not accepted and a failure reply is returned.  In sending a positive

   completion reply to the end of data indication, the receiver takes

   full responsibility for the message (see section 6.1).  Errors that

   are diagnosed subsequently MUST be reported in a mail message, as

   discussed in section 4.4.











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   When the SMTP server accepts a message either for relaying or for

   final delivery, it inserts a trace record (also referred to

   interchangeably as a "time stamp line" or "Received" line) at the top

   of the mail data.  This trace record indicates the identity of the

   host that sent the message, the identity of the host that received

   the message (and is inserting this time stamp), and the date and time

   the message was received.  Relayed messages will have multiple time

   stamp lines.  Details for formation of these lines, including their

   syntax, is specified in section 4.4.



   Additional discussion about the operation of the DATA command appears

   in section 3.3.



   Syntax:

      "DATA" CRLF



4.1.1.5 RESET (RSET)



   This command specifies that the current mail transaction will be

   aborted.  Any stored sender, recipients, and mail data MUST be

   discarded, and all buffers and state tables cleared.  The receiver

   MUST send a "250 OK" reply to a RSET command with no arguments.  A

   reset command may be issued by the client at any time.  It is

   effectively equivalent to a NOOP (i.e., if has no effect) if issued

   immediately after EHLO, before EHLO is issued in the session, after

   an end-of-data indicator has been sent and acknowledged, or

   immediately before a QUIT.  An SMTP server MUST NOT close the

   connection as the result of receiving a RSET; that action is reserved

   for QUIT (see section 4.1.1.10).



   Since EHLO implies some additional processing and response by the

   server, RSET will normally be more efficient than reissuing that

   command, even though the formal semantics are the same.



   There are circumstances, contrary to the intent of this

   specification, in which an SMTP server may receive an indication that

   the underlying TCP connection has been closed or reset.  To preserve

   the robustness of the mail system, SMTP servers SHOULD be prepared

   for this condition and SHOULD treat it as if a QUIT had been received

   before the connection disappeared.



   Syntax:

      "RSET" CRLF

















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4.1.1.6 VERIFY (VRFY)



   This command asks the receiver to confirm that the argument

   identifies a user or mailbox.  If it is a user name, information is

   returned as specified in section 3.5.



   This command has no effect on the reverse-path buffer, the forward-

   path buffer, or the mail data buffer.



   Syntax:

      "VRFY" SP String CRLF



4.1.1.7 EXPAND (EXPN)



   This command asks the receiver to confirm that the argument

   identifies a mailing list, and if so, to return the membership of

   that list.  If the command is successful, a reply is returned

   containing information as described in section 3.5.  This reply will

   have multiple lines except in the trivial case of a one-member list.



   This command has no effect on the reverse-path buffer, the forward-

   path buffer, or the mail data buffer and may be issued at any time.



   Syntax:

      "EXPN" SP String CRLF



4.1.1.8 HELP (HELP)



   This command causes the server to send helpful information to the

   client.  The command MAY take an argument (e.g., any command name)

   and return more specific information as a response.



   This command has no effect on the reverse-path buffer, the forward-

   path buffer, or the mail data buffer and may be issued at any time.



   SMTP servers SHOULD support HELP without arguments and MAY support it

   with arguments.



   Syntax:

      "HELP" [ SP String ] CRLF



4.1.1.9 NOOP (NOOP)



   This command does not affect any parameters or previously entered

   commands.  It specifies no action other than that the receiver send

   an OK reply.











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   This command has no effect on the reverse-path buffer, the forward-

   path buffer, or the mail data buffer and may be issued at any time.

   If a parameter string is specified, servers SHOULD ignore it.



   Syntax:

      "NOOP" [ SP String ] CRLF



4.1.1.10 QUIT (QUIT)



   This command specifies that the receiver MUST send an OK reply, and

   then close the transmission channel.



   The receiver MUST NOT intentionally close the transmission channel

   until it receives and replies to a QUIT command (even if there was an

   error).  The sender MUST NOT intentionally close the transmission

   channel until it sends a QUIT command and SHOULD wait until it

   receives the reply (even if there was an error response to a previous

   command).  If the connection is closed prematurely due to violations

   of the above or system or network failure, the server MUST cancel any

   pending transaction, but not undo any previously completed

   transaction, and generally MUST act as if the command or transaction

   in progress had received a temporary error (i.e., a 4yz response).



   The QUIT command may be issued at any time.



   Syntax:

      "QUIT" CRLF



4.1.2 Command Argument Syntax



   The syntax of the argument fields of the above commands (using the

   syntax specified in [8] where applicable) is given below.  Some of

   the productions given below are used only in conjunction with source

   routes as described in appendix C.  Terminals not defined in this

   document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined in

   the "core" syntax [8 (section 6)] or in the message format syntax

   [32].



      Reverse-path = Path

      Forward-path = Path

      Path = "<" [ A-d-l ":" ] Mailbox ">"

      A-d-l = At-domain *( "," A-d-l )

            ; Note that this form, the so-called "source route",

            ; MUST BE accepted, SHOULD NOT be generated, and SHOULD be

            ; ignored.

      At-domain = "@" domain

      Mail-parameters = esmtp-param *(SP esmtp-param)

      Rcpt-parameters = esmtp-param *(SP esmtp-param)







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      esmtp-param     = esmtp-keyword ["=" esmtp-value]

      esmtp-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")

      esmtp-value     = 1*(%d33-60 / %d62-127)

            ; any CHAR excluding "=", SP, and control characters

      Keyword  = Ldh-str

      Argument = Atom

      Domain = (sub-domain 1*("." sub-domain)) / address-literal

      sub-domain = Let-dig [Ldh-str]



      address-literal = "[" IPv4-address-literal /

                            IPv6-address-literal /

                            General-address-literal "]"

            ; See section 4.1.3



      Mailbox = Local-part "@" Domain



      Local-part = Dot-string / Quoted-string

            ; MAY be case-sensitive



      Dot-string = Atom *("." Atom)



      Atom = 1*atext



      Quoted-string = DQUOTE *qcontent DQUOTE



      String = Atom / Quoted-string



   While the above definition for Local-part is relatively permissive,

   for maximum interoperability, a host that expects to receive mail

   SHOULD avoid defining mailboxes where the Local-part requires (or

   uses) the Quoted-string form or where the Local-part is case-

   sensitive.  For any purposes that require generating or comparing

   Local-parts (e.g., to specific mailbox names), all quoted forms MUST

   be treated as equivalent and the sending system SHOULD transmit the

   form that uses the minimum quoting possible.



   Systems MUST NOT define mailboxes in such a way as to require the use

   in SMTP of non-ASCII characters (octets with the high order bit set

   to one) or ASCII "control characters" (decimal value 0-31 and 127).

   These characters MUST NOT be used in MAIL or RCPT commands or other

   commands that require mailbox names.



   Note that the backslash, "\", is a quote character, which is used to

   indicate that the next character is to be used literally (instead of

   its normal interpretation).  For example, "Joe\,Smith" indicates a

   single nine character user field with the comma being the fourth

   character of the field.









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   To promote interoperability and consistent with long-standing

   guidance about conservative use of the DNS in naming and applications

   (e.g., see section 2.3.1 of the base DNS document, RFC1035 [22]),

   characters outside the set of alphas, digits, and hyphen MUST NOT

   appear in domain name labels for SMTP clients or servers.  In

   particular, the underscore character is not permitted.  SMTP servers

   that receive a command in which invalid character codes have been

   employed, and for which there are no other reasons for rejection,

   MUST reject that command with a 501 response.



4.1.3 Address Literals



   Sometimes a host is not known to the domain name system and

   communication (and, in particular, communication to report and repair

   the error) is blocked.  To bypass this barrier a special literal form

   of the address is allowed as an alternative to a domain name.  For

   IPv4 addresses, this form uses four small decimal integers separated

   by dots and enclosed by brackets such as [123.255.37.2], which

   indicates an (IPv4) Internet Address in sequence-of-octets form.  For

   IPv6 and other forms of addressing that might eventually be

   standardized, the form consists of a standardized "tag" that

   identifies the address syntax, a colon, and the address itself, in a

   format specified as part of the IPv6 standards [17].



   Specifically:



      IPv4-address-literal = Snum 3("." Snum)

      IPv6-address-literal = "IPv6:" IPv6-addr

      General-address-literal = Standardized-tag ":" 1*dcontent

      Standardized-tag = Ldh-str

            ; MUST be specified in a standards-track RFC

            ; and registered with IANA



      Snum = 1*3DIGIT  ; representing a decimal integer

            ; value in the range 0 through 255

      Let-dig = ALPHA / DIGIT

      Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig



      IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp

      IPv6-hex  = 1*4HEXDIG

      IPv6-full = IPv6-hex 7(":" IPv6-hex)

      IPv6-comp = [IPv6-hex *5(":" IPv6-hex)] "::" [IPv6-hex *5(":"

                 IPv6-hex)]

            ; The "::" represents at least 2 16-bit groups of zeros

            ; No more than 6 groups in addition to the "::" may be

            ; present

      IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal

      IPv6v4-comp = [IPv6-hex *3(":" IPv6-hex)] "::"







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                   [IPv6-hex *3(":" IPv6-hex) ":"] IPv4-address-literal

            ; The "::" represents at least 2 16-bit groups of zeros

            ; No more than 4 groups in addition to the "::" and

            ; IPv4-address-literal may be present



4.1.4 Order of Commands



   There are restrictions on the order in which these commands may be

   used.



   A session that will contain mail transactions MUST first be

   initialized by the use of the EHLO command.  An SMTP server SHOULD

   accept commands for non-mail transactions (e.g., VRFY or EXPN)

   without this initialization.



   An EHLO command MAY be issued by a client later in the session.  If

   it is issued after the session begins, the SMTP server MUST clear all

   buffers and reset the state exactly as if a RSET command had been

   issued.  In other words, the sequence of RSET followed immediately by

   EHLO is redundant, but not harmful other than in the performance cost

   of executing unnecessary commands.



   If the EHLO command is not acceptable to the SMTP server, 501, 500,

   or 502 failure replies MUST be returned as appropriate.  The SMTP

   server MUST stay in the same state after transmitting these replies

   that it was in before the EHLO was received.



   The SMTP client MUST, if possible, ensure that the domain parameter

   to the EHLO command is a valid principal host name (not a CNAME or MX

   name) for its host.  If this is not possible (e.g., when the client's

   address is dynamically assigned and the client does not have an

   obvious name), an address literal SHOULD be substituted for the

   domain name and supplemental information provided that will assist in

   identifying the client.



   An SMTP server MAY verify that the domain name parameter in the EHLO

   command actually corresponds to the IP address of the client.

   However, the server MUST NOT refuse to accept a message for this

   reason if the verification fails: the information about verification

   failure is for logging and tracing only.



   The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time

   during a session, or without previously initializing a session.  SMTP

   servers SHOULD process these normally (that is, not return a 503

   code) even if no EHLO command has yet been received; clients SHOULD

   open a session with EHLO before sending these commands.











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   If these rules are followed, the example in RFC 821 that shows "550

   access denied to you" in response to an EXPN command is incorrect

   unless an EHLO command precedes the EXPN or the denial of access is

   based on the client's IP address or other authentication or

   authorization-determining mechanisms.



   The MAIL command (or the obsolete SEND, SOML, or SAML commands)

   begins a mail transaction.  Once started, a mail transaction consists

   of a transaction beginning command, one or more RCPT commands, and a

   DATA command, in that order.  A mail transaction may be aborted by

   the RSET (or a new EHLO) command.  There may be zero or more

   transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be

   sent if a mail transaction is already open, i.e., it should be sent

   only if no mail transaction had been started in the session, or it

   the previous one successfully concluded with a successful DATA

   command, or if the previous one was aborted with a RSET.



   If the transaction beginning command argument is not acceptable, a

   501 failure reply MUST be returned and the SMTP server MUST stay in

   the same state.  If the commands in a transaction are out of order to

   the degree that they cannot be processed by the server, a 503 failure

   reply MUST be returned and the SMTP server MUST stay in the same

   state.



   The last command in a session MUST be the QUIT command.  The QUIT

   command cannot be used at any other time in a session, but SHOULD be

   used by the client SMTP to request connection closure, even when no

   session opening command was sent and accepted.



4.1.5 Private-use Commands



   As specified in section 2.2.2, commands starting in "X" may be used

   by bilateral agreement between the client (sending) and server

   (receiving) SMTP agents.  An SMTP server that does not recognize such

   a command is expected to reply with "500 Command not recognized".  An

   extended SMTP server MAY list the feature names associated with these

   private commands in the response to the EHLO command.



   Commands sent or accepted by SMTP systems that do not start with "X"

   MUST conform to the requirements of section 2.2.2.



4.2 SMTP Replies



   Replies to SMTP commands serve to ensure the synchronization of

   requests and actions in the process of mail transfer and to guarantee

   that the SMTP client always knows the state of the SMTP server.

   Every command MUST generate exactly one reply.









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   The details of the command-reply sequence are described in section

   4.3.



   An SMTP reply consists of a three digit number (transmitted as three

   numeric characters) followed by some text unless specified otherwise

   in this document.  The number is for use by automata to determine

   what state to enter next; the text is for the human user.  The three

   digits contain enough encoded information that the SMTP client need

   not examine the text and may either discard it or pass it on to the

   user, as appropriate.  Exceptions are as noted elsewhere in this

   document.  In particular, the 220, 221, 251, 421, and 551 reply codes

   are associated with message text that must be parsed and interpreted

   by machines.  In the general case, the text may be receiver dependent

   and context dependent, so there are likely to be varying texts for

   each reply code.  A discussion of the theory of reply codes is given

   in section 4.2.1.  Formally, a reply is defined to be the sequence: a

   three-digit code, <SP>, one line of text, and <CRLF>, or a multiline

   reply (as defined in section 4.2.1).  Since, in violation of this

   specification, the text is sometimes not sent, clients which do not

   receive it SHOULD be prepared to process the code alone (with or

   without a trailing space character).  Only the EHLO, EXPN, and HELP

   commands are expected to result in multiline replies in normal

   circumstances, however, multiline replies are allowed for any

   command.



   In ABNF, server responses are:



      Greeting = "220 " Domain [ SP text ] CRLF

      Reply-line = Reply-code [ SP text ] CRLF



   where "Greeting" appears only in the 220 response that announces that

   the server is opening its part of the connection.



   An SMTP server SHOULD send only the reply codes listed in this

   document.  An SMTP server SHOULD use the text shown in the examples

   whenever appropriate.



   An SMTP client MUST determine its actions only by the reply code, not

   by the text (except for the "change of address" 251 and 551 and, if

   necessary, 220, 221, and 421 replies); in the general case, any text,

   including no text at all (although senders SHOULD NOT send bare

   codes), MUST be acceptable.  The space (blank) following the reply

   code is considered part of the text.  Whenever possible, a receiver-

   SMTP SHOULD test the first digit (severity indication) of the reply

   code.













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   The list of codes that appears below MUST NOT be construed as

   permanent.  While the addition of new codes should be a rare and

   significant activity, with supplemental information in the textual

   part of the response being preferred, new codes may be added as the

   result of new Standards or Standards-track specifications.

   Consequently, a sender-SMTP MUST be prepared to handle codes not

   specified in this document and MUST do so by interpreting the first

   digit only.



4.2.1 Reply Code Severities and Theory



   The three digits of the reply each have a special significance.  The

   first digit denotes whether the response is good, bad or incomplete.

   An unsophisticated SMTP client, or one that receives an unexpected

   code, will be able to determine its next action (proceed as planned,

   redo, retrench, etc.) by examining this first digit.  An SMTP client

   that wants to know approximately what kind of error occurred (e.g.,

   mail system error, command syntax error) may examine the second

   digit.  The third digit and any supplemental information that may be

   present is reserved for the finest gradation of information.



   There are five values for the first digit of the reply code:



   1yz   Positive Preliminary reply

      The command has been accepted, but the requested action is being

      held in abeyance, pending confirmation of the information in this

      reply.  The SMTP client should send another command specifying

      whether to continue or abort the action.  Note: unextended SMTP

      does not have any commands that allow this type of reply, and so

      does not have continue or abort commands.



   2yz   Positive Completion reply

      The requested action has been successfully completed.  A new

      request may be initiated.



   3yz   Positive Intermediate reply

      The command has been accepted, but the requested action is being

      held in abeyance, pending receipt of further information.  The

      SMTP client should send another command specifying this

      information.  This reply is used in command sequence groups (i.e.,

      in DATA).



   4yz   Transient Negative Completion reply

      The command was not accepted, and the requested action did not

      occur.  However, the error condition is temporary and the action

      may be requested again.  The sender should return to the beginning

      of the command sequence (if any).  It is difficult to assign a

      meaning to "transient" when two different sites (receiver- and







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      sender-SMTP agents) must agree on the interpretation.  Each reply

      in this category might have a different time value, but the SMTP

      client is encouraged to try again.  A rule of thumb to determine

      whether a reply fits into the 4yz or the 5yz category (see below)

      is that replies are 4yz if they can be successful if repeated

      without any change in command form or in properties of the sender

      or receiver (that is, the command is repeated identically and the

      receiver does not put up a new implementation.)



   5yz   Permanent Negative Completion reply

      The command was not accepted and the requested action did not

      occur.  The SMTP client is discouraged from repeating the exact

      request (in the same sequence).  Even some "permanent" error

      conditions can be corrected, so the human user may want to direct

      the SMTP client to reinitiate the command sequence by direct

      action at some point in the future (e.g., after the spelling has

      been changed, or the user has altered the account status).



   The second digit encodes responses in specific categories:



   x0z   Syntax: These replies refer to syntax errors, syntactically

      correct commands that do not fit any functional category, and

      unimplemented or superfluous commands.



   x1z   Information:  These are replies to requests for information,

      such as status or help.



   x2z   Connections: These are replies referring to the transmission

      channel.



   x3z   Unspecified.



   x4z   Unspecified.



   x5z   Mail system: These replies indicate the status of the receiver

      mail system vis-a-vis the requested transfer or other mail system

      action.



   The third digit gives a finer gradation of meaning in each category

   specified by the second digit.  The list of replies illustrates this.

   Each reply text is recommended rather than mandatory, and may even

   change according to the command with which it is associated.  On the

   other hand, the reply codes must strictly follow the specifications

   in this section.  Receiver implementations should not invent new

   codes for slightly different situations from the ones described here,

   but rather adapt codes already defined.











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   For example, a command such as NOOP, whose successful execution does

   not offer the SMTP client any new information, will return a 250

   reply.  The reply is 502 when the command requests an unimplemented

   non-site-specific action.  A refinement of that is the 504 reply for

   a command that is implemented, but that requests an unimplemented

   parameter.



   The reply text may be longer than a single line; in these cases the

   complete text must be marked so the SMTP client knows when it can

   stop reading the reply.  This requires a special format to indicate a

   multiple line reply.



   The format for multiline replies requires that every line, except the

   last, begin with the reply code, followed immediately by a hyphen,

   "-" (also known as minus), followed by text.  The last line will

   begin with the reply code, followed immediately by <SP>, optionally

   some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>

   if subsequent text is not sent, but clients MUST be prepared for it

   to be omitted.



   For example:



      123-First line

      123-Second line

      123-234 text beginning with numbers

      123 The last line



   In many cases the SMTP client then simply needs to search for a line

   beginning with the reply code followed by <SP> or <CRLF> and ignore

   all preceding lines.  In a few cases, there is important data for the

   client in the reply "text".  The client will be able to identify

   these cases from the current context.



4.2.2 Reply Codes by Function Groups



      500 Syntax error, command unrecognized

         (This may include errors such as command line too long)

      501 Syntax error in parameters or arguments

      502 Command not implemented  (see section 4.2.4)

      503 Bad sequence of commands

      504 Command parameter not implemented



      211 System status, or system help reply

      214 Help message

         (Information on how to use the receiver or the meaning of a

         particular non-standard command; this reply is useful only

         to the human user)









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      220 <domain> Service ready

      221 <domain> Service closing transmission channel

      421 <domain> Service not available, closing transmission channel

         (This may be a reply to any command if the service knows it

         must shut down)



      250 Requested mail action okay, completed

      251 User not local; will forward to <forward-path>

         (See section 3.4)

      252 Cannot VRFY user, but will accept message and attempt

          delivery

         (See section 3.5.3)

      450 Requested mail action not taken: mailbox unavailable

         (e.g., mailbox busy)

      550 Requested action not taken: mailbox unavailable

         (e.g., mailbox not found, no access, or command rejected

         for policy reasons)

      451 Requested action aborted: error in processing

      551 User not local; please try <forward-path>

         (See section 3.4)

      452 Requested action not taken: insufficient system storage

      552 Requested mail action aborted: exceeded storage allocation

      553 Requested action not taken: mailbox name not allowed

         (e.g., mailbox syntax incorrect)

      354 Start mail input; end with <CRLF>.<CRLF>

      554 Transaction failed (Or, in the case of a connection-opening

          response, "No SMTP service here")



4.2.3  Reply Codes in Numeric Order



      211 System status, or system help reply

      214 Help message

         (Information on how to use the receiver or the meaning of a

         particular non-standard command; this reply is useful only

         to the human user)

      220 <domain> Service ready

      221 <domain> Service closing transmission channel

      250 Requested mail action okay, completed

      251 User not local; will forward to <forward-path>

         (See section 3.4)

      252 Cannot VRFY user, but will accept message and attempt

         delivery

         (See section 3.5.3)



      354 Start mail input; end with <CRLF>.<CRLF>













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      421 <domain> Service not available, closing transmission channel

         (This may be a reply to any command if the service knows it

         must shut down)

      450 Requested mail action not taken: mailbox unavailable

         (e.g., mailbox busy)

      451 Requested action aborted: local error in processing

      452 Requested action not taken: insufficient system storage

      500 Syntax error, command unrecognized

         (This may include errors such as command line too long)

      501 Syntax error in parameters or arguments

      502 Command not implemented (see section 4.2.4)

      503 Bad sequence of commands

      504 Command parameter not implemented

      550 Requested action not taken: mailbox unavailable

         (e.g., mailbox not found, no access, or command rejected

         for policy reasons)

      551 User not local; please try <forward-path>

         (See section 3.4)

      552 Requested mail action aborted: exceeded storage allocation

      553 Requested action not taken: mailbox name not allowed

         (e.g., mailbox syntax incorrect)

      554 Transaction failed  (Or, in the case of a connection-opening

          response, "No SMTP service here")



4.2.4 Reply Code 502



   Questions have been raised as to when reply code 502 (Command not

   implemented) SHOULD be returned in preference to other codes.  502

   SHOULD be used when the command is actually recognized by the SMTP

   server, but not implemented.  If the command is not recognized, code

   500 SHOULD be returned.  Extended SMTP systems MUST NOT list

   capabilities in response to EHLO for which they will return 502 (or

   500) replies.



4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>



   When an SMTP server returns a positive completion status (2yz code)

   after the DATA command is completed with <CRLF>.<CRLF>, it accepts

   responsibility for:



   -  delivering the message (if the recipient mailbox exists), or



   -  if attempts to deliver the message fail due to transient

      conditions, retrying delivery some reasonable number of times at

      intervals as specified in section 4.5.4.













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   -  if attempts to deliver the message fail due to permanent

      conditions, or if repeated attempts to deliver the message fail

      due to transient conditions, returning appropriate notification to

      the sender of the original message (using the address in the SMTP

      MAIL command).



   When an SMTP server returns a permanent error status (5yz) code after

   the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make

   any subsequent attempt to deliver that message.  The SMTP client

   retains responsibility for delivery of that message and may either

   return it to the user or requeue it for a subsequent attempt (see

   section 4.5.4.1).



   The user who originated the message SHOULD be able to interpret the

   return of a transient failure status (by mail message or otherwise)

   as a non-delivery indication, just as a permanent failure would be

   interpreted.  I.e., if the client SMTP successfully handles these

   conditions, the user will not receive such a reply.



   When an SMTP server returns a permanent error status (5yz) code after

   the DATA command is completely with <CRLF>.<CRLF>, it MUST NOT make

   any subsequent attempt to deliver the message.  As with temporary

   error status codes, the SMTP client retains responsibility for the

   message, but SHOULD not again attempt delivery to the same server

   without user review and intervention of the message.



4.3 Sequencing of Commands and Replies



4.3.1 Sequencing Overview



   The communication between the sender and receiver is an alternating

   dialogue, controlled by the sender.  As such, the sender issues a

   command and the receiver responds with a reply.  Unless other

   arrangements are negotiated through service extensions, the sender

   MUST wait for this response before sending further commands.



   One important reply is the connection greeting.  Normally, a receiver

   will send a 220 "Service ready" reply when the connection is

   completed.  The sender SHOULD wait for this greeting message before

   sending any commands.



   Note: all the greeting-type replies have the official name (the

   fully-qualified primary domain name) of the server host as the first

   word following the reply code.  Sometimes the host will have no

   meaningful name.  See 4.1.3 for a discussion of alternatives in these

   situations.











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   For example,



      220 ISIF.USC.EDU Service ready

   or

      220 mail.foo.com SuperSMTP v 6.1.2 Service ready

   or

      220 [10.0.0.1] Clueless host service ready



   The table below lists alternative success and failure replies for

   each command.  These SHOULD be strictly adhered to: a receiver may

   substitute text in the replies, but the meaning and action implied by

   the code numbers and by the specific command reply sequence cannot be

   altered.



4.3.2 Command-Reply Sequences



   Each command is listed with its usual possible replies.  The prefixes

   used before the possible replies are "I" for intermediate, "S" for

   success, and "E" for error.  Since some servers may generate other

   replies under special circumstances, and to allow for future

   extension, SMTP clients SHOULD, when possible, interpret only the

   first digit of the reply and MUST be prepared to deal with

   unrecognized reply codes by interpreting the first digit only.

   Unless extended using the mechanisms described in section 2.2, SMTP

   servers MUST NOT transmit reply codes to an SMTP client that are

   other than three digits or that do not start in a digit between 2 and

   5 inclusive.



   These sequencing rules and, in principle, the codes themselves, can

   be extended or modified by SMTP extensions offered by the server and

   accepted (requested) by the client.



   In addition to the codes listed below, any SMTP command can return

   any of the following codes if the corresponding unusual circumstances

   are encountered:



   500  For the "command line too long" case or if the command name was

      not recognized.  Note that producing a "command not recognized"

      error in response to the required subset of these commands is a

      violation of this specification.



   501  Syntax error in command or arguments.  In order to provide for

      future extensions, commands that are specified in this document as

      not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501

      message if arguments are supplied in the absence of EHLO-

      advertised extensions.



   421  Service shutting down and closing transmission channel







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   Specific sequences are:



   CONNECTION ESTABLISHMENT

      S: 220

      E: 554

   EHLO or HELO

      S: 250

      E: 504, 550

   MAIL

      S: 250

      E: 552, 451, 452, 550, 553, 503

   RCPT

      S: 250, 251 (but see section 3.4 for discussion of 251 and 551)

      E: 550, 551, 552, 553, 450, 451, 452, 503, 550

   DATA

      I: 354 -> data -> S: 250

                        E: 552, 554, 451, 452

      E: 451, 554, 503

   RSET

      S: 250

   VRFY

      S: 250, 251, 252

      E: 550, 551, 553, 502, 504

   EXPN

      S: 250, 252

      E: 550, 500, 502, 504

   HELP

      S: 211, 214

      E: 502, 504

   NOOP

      S: 250

   QUIT

      S: 221



4.4 Trace Information



   When an SMTP server receives a message for delivery or further

   processing, it MUST insert trace ("time stamp" or "Received")

   information at the beginning of the message content, as discussed in

   section 4.1.1.4.



   This line MUST be structured as follows:



   -  The FROM field, which MUST be supplied in an SMTP environment,

      SHOULD contain both (1) the name of the source host as presented

      in the EHLO command and (2) an address literal containing the IP

      address of the source, determined from the TCP connection.









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   -  The ID field MAY contain an "@" as suggested in RFC 822, but this

      is not required.



   -  The FOR field MAY contain a list of <path> entries when multiple

      RCPT commands have been given.  This may raise some security

      issues and is usually not desirable; see section 7.2.



   An Internet mail program MUST NOT change a Received: line that was

   previously added to the message header.  SMTP servers MUST prepend

   Received lines to messages; they MUST NOT change the order of

   existing lines or insert Received lines in any other location.



   As the Internet grows, comparability of Received fields is important

   for detecting problems, especially slow relays.  SMTP servers that

   create Received fields SHOULD use explicit offsets in the dates

   (e.g., -0800), rather than time zone names of any type.  Local time

   (with an offset) is preferred to UT when feasible.  This formulation

   allows slightly more information about local circumstances to be

   specified.  If UT is needed, the receiver need merely do some simple

   arithmetic to convert the values.  Use of UT loses information about

   the time zone-location of the server.  If it is desired to supply a

   time zone name, it SHOULD be included in a comment.



   When the delivery SMTP server makes the "final delivery" of a

   message, it inserts a return-path line at the beginning of the mail

   data.  This use of return-path is required; mail systems MUST support

   it.  The return-path line preserves the information in the <reverse-

   path> from the MAIL command.  Here, final delivery means the message

   has left the SMTP environment.  Normally, this would mean it had been

   delivered to the destination user or an associated mail drop, but in

   some cases it may be further processed and transmitted by another

   mail system.



   It is possible for the mailbox in the return path to be different

   from the actual sender's mailbox, for example, if error responses are

   to be delivered to a special error handling mailbox rather than to

   the message sender.  When mailing lists are involved, this

   arrangement is common and useful as a means of directing errors to

   the list maintainer rather than the message originator.



   The text above implies that the final mail data will begin with a

   return path line, followed by one or more time stamp lines.  These

   lines will be followed by the mail data headers and body [32].



   It is sometimes difficult for an SMTP server to determine whether or

   not it is making final delivery since forwarding or other operations

   may occur after the message is accepted for delivery.  Consequently,









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   any further (forwarding, gateway, or relay) systems MAY remove the

   return path and rebuild the MAIL command as needed to ensure that

   exactly one such line appears in a delivered message.



   A message-originating SMTP system SHOULD NOT send a message that

   already contains a Return-path header.  SMTP servers performing a

   relay function MUST NOT inspect the message data, and especially not

   to the extent needed to determine if Return-path headers are present.

   SMTP servers making final delivery MAY remove Return-path headers

   before adding their own.



   The primary purpose of the Return-path is to designate the address to

   which messages indicating non-delivery or other mail system failures

   are to be sent.  For this to be unambiguous, exactly one return path

   SHOULD be present when the message is delivered.  Systems using RFC

   822 syntax with non-SMTP transports SHOULD designate an unambiguous

   address, associated with the transport envelope, to which error

   reports (e.g., non-delivery messages) should be sent.



   Historical note: Text in RFC 822 that appears to contradict the use

   of the Return-path header (or the envelope reverse path address from

   the MAIL command) as the destination for error messages is not

   applicable on the Internet.  The reverse path address (as copied into

   the Return-path) MUST be used as the target of any mail containing

   delivery error messages.



   In particular:



   -  a gateway from SMTP->elsewhere SHOULD insert a return-path header,

      unless it is known that the "elsewhere" transport also uses

      Internet domain addresses and maintains the envelope sender

      address separately.



   -  a gateway from elsewhere->SMTP SHOULD delete any return-path

      header present in the message, and either copy that information to

      the SMTP envelope or combine it with information present in the

      envelope of the other transport system to construct the reverse

      path argument to the MAIL command in the SMTP envelope.



   The server must give special treatment to cases in which the

   processing following the end of mail data indication is only

   partially successful.  This could happen if, after accepting several

   recipients and the mail data, the SMTP server finds that the mail

   data could be successfully delivered to some, but not all, of the

   recipients.  In such cases, the response to the DATA command MUST be

   an OK reply.  However, the SMTP server MUST compose and send an

   "undeliverable mail" notification message to the originator of the

   message.







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   A single notification listing all of the failed recipients or

   separate notification messages MUST be sent for each failed

   recipient.  For economy of processing by the sender, the former is

   preferred when possible.  All undeliverable mail notification

   messages are sent using the MAIL command (even if they result from

   processing the obsolete SEND, SOML, or SAML commands) and use a null

   return path as discussed in section 3.7.



   The time stamp line and the return path line are formally defined as

   follows:



Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>



Time-stamp-line = "Received:" FWS Stamp <CRLF>



Stamp = From-domain By-domain Opt-info ";"  FWS date-time



      ; where "date-time" is as defined in [32]

      ; but the "obs-" forms, especially two-digit

      ; years, are prohibited in SMTP and MUST NOT be used.



From-domain = "FROM" FWS Extended-Domain CFWS



By-domain = "BY" FWS Extended-Domain CFWS



Extended-Domain = Domain /

           ( Domain FWS "(" TCP-info ")" ) /

           ( Address-literal FWS "(" TCP-info ")" )



TCP-info = Address-literal / ( Domain FWS Address-literal )

      ; Information derived by server from TCP connection

      ; not client EHLO.



Opt-info = [Via] [With] [ID] [For]



Via = "VIA" FWS Link CFWS



With = "WITH" FWS Protocol CFWS



ID = "ID" FWS String / msg-id CFWS



For = "FOR" FWS 1*( Path / Mailbox ) CFWS



Link = "TCP" / Addtl-Link

Addtl-Link = Atom

      ; Additional standard names for links are registered with the

         ; Internet Assigned Numbers Authority (IANA).  "Via" is

         ; primarily of value with non-Internet transports.  SMTP







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         ; servers SHOULD NOT use unregistered names.

Protocol = "ESMTP" / "SMTP" / Attdl-Protocol

Attdl-Protocol = Atom

      ; Additional standard names for protocols are registered with the

         ; Internet Assigned Numbers Authority (IANA).  SMTP servers

         ; SHOULD NOT use unregistered names.



4.5 Additional Implementation Issues



4.5.1 Minimum Implementation



   In order to make SMTP workable, the following minimum implementation

   is required for all receivers.  The following commands MUST be

   supported to conform to this specification:



      EHLO

      HELO

      MAIL

      RCPT

      DATA

      RSET

      NOOP

      QUIT

      VRFY



   Any system that includes an SMTP server supporting mail relaying or

   delivery MUST support the reserved mailbox "postmaster" as a case-

   insensitive local name.  This postmaster address is not strictly

   necessary if the server always returns 554 on connection opening (as

   described in section 3.1).  The requirement to accept mail for

   postmaster implies that RCPT commands which specify a mailbox for

   postmaster at any of the domains for which the SMTP server provides

   mail service, as well as the special case of "RCPT TO:<Postmaster>"

   (with no domain specification), MUST be supported.



   SMTP systems are expected to make every reasonable effort to accept

   mail directed to Postmaster from any other system on the Internet.

   In extreme cases --such as to contain a denial of service attack or

   other breach of security-- an SMTP server may block mail directed to

   Postmaster.  However, such arrangements SHOULD be narrowly tailored

   so as to avoid blocking messages which are not part of such attacks.



4.5.2 Transparency



   Without some provision for data transparency, the character sequence

   "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.

   In general, users are not aware of such "forbidden" sequences.  To









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   allow all user composed text to be transmitted transparently, the

   following procedures are used:



   -  Before sending a line of mail text, the SMTP client checks the

      first character of the line.  If it is a period, one additional

      period is inserted at the beginning of the line.



   -  When a line of mail text is received by the SMTP server, it checks

      the line.  If the line is composed of a single period, it is

      treated as the end of mail indicator.  If the first character is a

      period and there are other characters on the line, the first

      character is deleted.



   The mail data may contain any of the 128 ASCII characters.  All

   characters are to be delivered to the recipient's mailbox, including

   spaces, vertical and horizontal tabs, and other control characters.

   If the transmission channel provides an 8-bit byte (octet) data

   stream, the 7-bit ASCII codes are transmitted right justified in the

   octets, with the high order bits cleared to zero.  See 3.7 for

   special treatment of these conditions in SMTP systems serving a relay

   function.



   In some systems it may be necessary to transform the data as it is

   received and stored.  This may be necessary for hosts that use a

   different character set than ASCII as their local character set, that

   store data in records rather than strings, or which use special

   character sequences as delimiters inside mailboxes.  If such

   transformations are necessary, they MUST be reversible, especially if

   they are applied to mail being relayed.



4.5.3 Sizes and Timeouts



4.5.3.1 Size limits and minimums



   There are several objects that have required minimum/maximum sizes.

   Every implementation MUST be able to receive objects of at least

   these sizes.  Objects larger than these sizes SHOULD be avoided when

   possible.  However, some Internet mail constructs such as encoded

   X.400 addresses [16] will often require larger objects: clients MAY

   attempt to transmit these, but MUST be prepared for a server to

   reject them if they cannot be handled by it.  To the maximum extent

   possible, implementation techniques which impose no limits on the

   length of these objects should be used.



   local-part

      The maximum total length of a user name or other local-part is 64

      characters.









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   domain

      The maximum total length of a domain name or number is 255

      characters.



   path

      The maximum total length of a reverse-path or forward-path is 256

      characters (including the punctuation and element separators).



   command line

      The maximum total length of a command line including the command

      word and the <CRLF> is 512 characters.  SMTP extensions may be

      used to increase this limit.



   reply line

      The maximum total length of a reply line including the reply code

      and the <CRLF> is 512 characters.  More information may be

      conveyed through multiple-line replies.



   text line

      The maximum total length of a text line including the <CRLF> is

      1000 characters (not counting the leading dot duplicated for

      transparency).  This number may be increased by the use of SMTP

      Service Extensions.



   message content

      The maximum total length of a message content (including any

      message headers as well as the message body) MUST BE at least 64K

      octets.  Since the introduction of Internet standards for

      multimedia mail [12], message lengths on the Internet have grown

      dramatically, and message size restrictions should be avoided if

      at all possible.  SMTP server systems that must impose

      restrictions SHOULD implement the "SIZE" service extension [18],

      and SMTP client systems that will send large messages SHOULD

      utilize it when possible.



   recipients buffer

      The minimum total number of recipients that must be buffered is

      100 recipients.  Rejection of messages (for excessive recipients)

      with fewer than 100 RCPT commands is a violation of this

      specification.  The general principle that relaying SMTP servers

      MUST NOT, and delivery SMTP servers SHOULD NOT, perform validation

      tests on message headers suggests that rejecting a message based

      on the total number of recipients shown in header fields is to be

      discouraged.  A server which imposes a limit on the number of

      recipients MUST behave in an orderly fashion,  such as to reject

      additional addresses over its limit rather than silently

      discarding addresses previously accepted.  A client that needs to









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      deliver a message containing over 100 RCPT commands SHOULD be

      prepared to transmit in 100-recipient "chunks" if the server

      declines to accept more than 100 recipients in a single message.



   Errors due to exceeding these limits may be reported by using the

   reply codes.  Some examples of reply codes are:



      500 Line too long.

   or

      501 Path too long

   or

      452 Too many recipients  (see below)

   or

      552 Too much mail data.



   RFC 821 [30] incorrectly listed the error where an SMTP server

   exhausts its implementation limit on the number of RCPT commands

   ("too many recipients") as having reply code 552.  The correct reply

   code for this condition is 452.  Clients SHOULD treat a 552 code in

   this case as a temporary, rather than permanent, failure so the logic

   below works.



   When a conforming SMTP server encounters this condition, it has at

   least 100 successful RCPT commands in its recipients buffer.  If the

   server is able to accept the message, then at least these 100

   addresses will be removed from the SMTP client's queue.  When the

   client attempts retransmission of those addresses which received 452

   responses, at least 100 of these will be able to fit in the SMTP

   server's recipients buffer.  Each retransmission attempt which is

   able to deliver anything will be able to dispose of at least 100 of

   these recipients.



   If an SMTP server has an implementation limit on the number of RCPT

   commands and this limit is exhausted, it MUST use a response code of

   452 (but the client SHOULD also be prepared for a 552, as noted

   above).  If the server has a configured site-policy limitation on the

   number of RCPT commands, it MAY instead use a 5XX response code.

   This would be most appropriate if the policy limitation was intended

   to apply if the total recipient count for a particular message body

   were enforced even if that message body was sent in multiple mail

   transactions.



4.5.3.2 Timeouts



   An SMTP client MUST provide a timeout mechanism.  It MUST use per-

   command timeouts rather than somehow trying to time the entire mail

   transaction.  Timeouts SHOULD be easily reconfigurable, preferably

   without recompiling the SMTP code.  To implement this, a timer is set







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   for each SMTP command and for each buffer of the data transfer.  The

   latter means that the overall timeout is inherently proportional to

   the size of the message.



   Based on extensive experience with busy mail-relay hosts, the minimum

   per-command timeout values SHOULD be as follows:



   Initial 220 Message: 5 minutes

      An SMTP client process needs to distinguish between a failed TCP

      connection and a delay in receiving the initial 220 greeting

      message.  Many SMTP servers accept a TCP connection but delay

      delivery of the 220 message until their system load permits more

      mail to be processed.



   MAIL Command: 5 minutes



   RCPT Command: 5 minutes

      A longer timeout is required if processing of mailing lists and

      aliases is not deferred until after the message was accepted.



   DATA Initiation: 2 minutes

      This is while awaiting the "354 Start Input" reply to a DATA

      command.



   Data Block: 3 minutes

      This is while awaiting the completion of each TCP SEND call

      transmitting a chunk of data.



   DATA Termination: 10 minutes.

      This is while awaiting the "250 OK" reply.  When the receiver gets

      the final period terminating the message data, it typically

      performs processing to deliver the message to a user mailbox.  A

      spurious timeout at this point would be very wasteful and would

      typically result in delivery of multiple copies of the message,

      since it has been successfully sent and the server has accepted

      responsibility for delivery.  See section 6.1 for additional

      discussion.



   An SMTP server SHOULD have a timeout of at least 5 minutes while it

   is awaiting the next command from the sender.



4.5.4 Retry Strategies



   The common structure of a host SMTP implementation includes user

   mailboxes, one or more areas for queuing messages in transit, and one

   or more daemon processes for sending and receiving mail.  The exact

   structure will vary depending on the needs of the users on the host









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   and the number and size of mailing lists supported by the host.  We

   describe several optimizations that have proved helpful, particularly

   for mailers supporting high traffic levels.



   Any queuing strategy MUST include timeouts on all activities on a

   per-command basis.  A queuing strategy MUST NOT send error messages

   in response to error messages under any circumstances.



4.5.4.1 Sending Strategy



   The general model for an SMTP client is one or more processes that

   periodically attempt to transmit outgoing mail.  In a typical system,

   the program that composes a message has some method for requesting

   immediate attention for a new piece of outgoing mail, while mail that

   cannot be transmitted immediately MUST be queued and periodically

   retried by the sender.  A mail queue entry will include not only the

   message itself but also the envelope information.



   The sender MUST delay retrying a particular destination after one

   attempt has failed.  In general, the retry interval SHOULD be at

   least 30 minutes; however, more sophisticated and variable strategies

   will be beneficial when the SMTP client can determine the reason for

   non-delivery.



   Retries continue until the message is transmitted or the sender gives

   up; the give-up time generally needs to be at least 4-5 days.  The

   parameters to the retry algorithm MUST be configurable.



   A client SHOULD keep a list of hosts it cannot reach and

   corresponding connection timeouts, rather than just retrying queued

   mail items.



   Experience suggests that failures are typically transient (the target

   system or its connection has crashed), favoring a policy of two

   connection attempts in the first hour the message is in the queue,

   and then backing off to one every two or three hours.



   The SMTP client can shorten the queuing delay in cooperation with the

   SMTP server.  For example, if mail is received from a particular

   address, it is likely that mail queued for that host can now be sent.

   Application of this principle may, in many cases, eliminate the

   requirement for an explicit "send queues now" function such as ETRN

   [9].



   The strategy may be further modified as a result of multiple

   addresses per host (see below) to optimize delivery time vs. resource

   usage.









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   An SMTP client may have a large queue of messages for each

   unavailable destination host.  If all of these messages were retried

   in every retry cycle, there would be excessive Internet overhead and

   the sending system would be blocked for a long period.  Note that an

   SMTP client can generally determine that a delivery attempt has

   failed only after a timeout of several minutes and even a one-minute

   timeout per connection will result in a very large delay if retries

   are repeated for dozens, or even hundreds, of queued messages to the

   same host.



   At the same time, SMTP clients SHOULD use great care in caching

   negative responses from servers.  In an extreme case, if EHLO is

   issued multiple times during the same SMTP connection, different

   answers may be returned by the server.  More significantly, 5yz

   responses to the MAIL command MUST NOT be cached.



   When a mail message is to be delivered to multiple recipients, and

   the SMTP server to which a copy of the message is to be sent is the

   same for multiple recipients, then only one copy of the message

   SHOULD be transmitted.  That is, the SMTP client SHOULD use the

   command sequence:  MAIL, RCPT, RCPT,... RCPT, DATA instead of the

   sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there

   are very many addresses, a limit on the number of RCPT commands per

   MAIL command MAY be imposed.  Implementation of this efficiency

   feature is strongly encouraged.



   Similarly, to achieve timely delivery, the SMTP client MAY support

   multiple concurrent outgoing mail transactions.  However, some limit

   may be appropriate to protect the host from devoting all its

   resources to mail.



4.5.4.2 Receiving Strategy



   The SMTP server SHOULD attempt to keep a pending listen on the SMTP

   port at all times.  This requires the support of multiple incoming

   TCP connections for SMTP.  Some limit MAY be imposed but servers that

   cannot handle more than one SMTP transaction at a time are not in

   conformance with the intent of this specification.



   As discussed above, when the SMTP server receives mail from a

   particular host address, it could activate its own SMTP queuing

   mechanisms to retry any mail pending for that host address.



4.5.5   Messages with a null reverse-path



   There are several types of notification messages which are required

   by existing and proposed standards to be sent with a null reverse

   path, namely non-delivery notifications as discussed in section 3.7,







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   other kinds of Delivery Status Notifications (DSNs) [24], and also

   Message Disposition Notifications (MDNs) [10].  All of these kinds of

   messages are notifications about a previous message, and they are

   sent to the reverse-path of the previous mail message.  (If the

   delivery of such a notification message fails, that usually indicates

   a problem with the mail system of the host to which the notification

   message is addressed.  For this reason, at some hosts the MTA is set

   up to forward such failed notification messages to someone who is

   able to fix problems with the mail system, e.g., via the postmaster

   alias.)



   All other types of messages (i.e., any message which is not required

   by a standards-track RFC to have a null reverse-path) SHOULD be sent

   with with a valid, non-null reverse-path.



   Implementors of automated email processors should be careful to make

   sure that the various kinds of messages with null reverse-path are

   handled correctly, in particular such systems SHOULD NOT reply to

   messages with null reverse-path.



5. Address Resolution and Mail Handling



   Once an SMTP client lexically identifies a domain to which mail will

   be delivered for processing (as described in sections 3.6 and 3.7), a

   DNS lookup MUST be performed to resolve the domain name [22].  The

   names are expected to be fully-qualified domain names (FQDNs):

   mechanisms for inferring FQDNs from partial names or local aliases

   are outside of this specification and, due to a history of problems,

   are generally discouraged.  The lookup first attempts to locate an MX

   record associated with the name.  If a CNAME record is found instead,

   the resulting name is processed as if it were the initial name.  If

   no MX records are found, but an A RR is found, the A RR is treated as

   if it was associated with an implicit MX RR, with a preference of 0,

   pointing to that host.  If one or more MX RRs are found for a given

   name, SMTP systems MUST NOT utilize any A RRs associated with that

   name unless they are located using the MX RRs; the "implicit MX" rule

   above applies only if there are no MX records present.  If MX records

   are present, but none of them are usable, this situation MUST be

   reported as an error.



   When the lookup succeeds, the mapping can result in a list of

   alternative delivery addresses rather than a single address, because

   of multiple MX records, multihoming, or both.  To provide reliable

   mail transmission, the SMTP client MUST be able to try (and retry)

   each of the relevant addresses in this list in order, until a

   delivery attempt succeeds.  However, there MAY also be a configurable

   limit on the number of alternate addresses that can be tried.  In any

   case, the SMTP client SHOULD try at least two addresses.







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   Two types of information is used to rank the host addresses: multiple

   MX records, and multihomed hosts.



   Multiple MX records contain a preference indication that MUST be used

   in sorting (see below).  Lower numbers are more preferred than higher

   ones.  If there are multiple destinations with the same preference

   and there is no clear reason to favor one (e.g., by recognition of an

   easily-reached address), then the sender-SMTP MUST randomize them to

   spread the load across multiple mail exchangers for a specific

   organization.



   The destination host (perhaps taken from the preferred MX record) may

   be multihomed, in which case the domain name resolver will return a

   list of alternative IP addresses.  It is the responsibility of the

   domain name resolver interface to have ordered this list by

   decreasing preference if necessary, and SMTP MUST try them in the

   order presented.



   Although the capability to try multiple alternative addresses is

   required, specific installations may want to limit or disable the use

   of alternative addresses.  The question of whether a sender should

   attempt retries using the different addresses of a multihomed host

   has been controversial.  The main argument for using the multiple

   addresses is that it maximizes the probability of timely delivery,

   and indeed sometimes the probability of any delivery; the counter-

   argument is that it may result in unnecessary resource use.  Note

   that resource use is also strongly determined by the sending strategy

   discussed in section 4.5.4.1.



   If an SMTP server receives a message with a destination for which it

   is a designated Mail eXchanger, it MAY relay the message (potentially

   after having rewritten the MAIL FROM and/or RCPT TO addresses), make

   final delivery of the message, or hand it off using some mechanism

   outside the SMTP-provided transport environment.  Of course, neither

   of the latter require that the list of MX records be examined

   further.



   If it determines that it should relay the message without rewriting

   the address, it MUST sort the MX records to determine candidates for

   delivery.  The records are first ordered by preference, with the

   lowest-numbered records being most preferred.  The relay host MUST

   then inspect the list for any of the names or addresses by which it

   might be known in mail transactions.  If a matching record is found,

   all records at that preference level and higher-numbered ones MUST be

   discarded from consideration.  If there are no records left at that

   point, it is an error condition, and the message MUST be returned as

   undeliverable.  If records do remain, they SHOULD be tried, best

   preference first, as described above.







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6. Problem Detection and Handling



6.1 Reliable Delivery and Replies by Email



   When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"

   message in response to DATA), it is accepting responsibility for

   delivering or relaying the message.  It must take this responsibility

   seriously.  It MUST NOT lose the message for frivolous reasons, such

   as because the host later crashes or because of a predictable

   resource shortage.



   If there is a delivery failure after acceptance of a message, the

   receiver-SMTP MUST formulate and mail a notification message.  This

   notification MUST be sent using a null ("<>") reverse path in the

   envelope.  The recipient of this notification MUST be the address

   from the envelope return path (or the Return-Path: line).  However,

   if this address is null ("<>"), the receiver-SMTP MUST NOT send a

   notification.  Obviously, nothing in this section can or should

   prohibit local decisions (i.e., as part of the same system

   environment as the receiver-SMTP) to log or otherwise transmit

   information about null address events locally if that is desired.  If

   the address is an explicit source route, it MUST be stripped down to

   its final hop.



   For example, suppose that an error notification must be sent for a

   message that arrived with:



      MAIL FROM:<@a,@b:user@d>



   The notification message MUST be sent using:



      RCPT TO:<user@d>



   Some delivery failures after the message is accepted by SMTP will be

   unavoidable.  For example, it may be impossible for the receiving

   SMTP server to validate all the delivery addresses in RCPT command(s)

   due to a "soft" domain system error, because the target is a mailing

   list (see earlier discussion of RCPT), or because the server is

   acting as a relay and has no immediate access to the delivering

   system.



   To avoid receiving duplicate messages as the result of timeouts, a

   receiver-SMTP MUST seek to minimize the time required to respond to

   the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [28] for

   a discussion of this problem.













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6.2 Loop Detection



   Simple counting of the number of "Received:" headers in a message has

   proven to be an effective, although rarely optimal, method of

   detecting loops in mail systems.  SMTP servers using this technique

   SHOULD use a large rejection threshold, normally at least 100

   Received entries.  Whatever mechanisms are used, servers MUST contain

   provisions for detecting and stopping trivial loops.



6.3 Compensating for Irregularities



   Unfortunately, variations, creative interpretations, and outright

   violations of Internet mail protocols do occur; some would suggest

   that they occur quite frequently.  The debate as to whether a well-

   behaved SMTP receiver or relay should reject a malformed message,

   attempt to pass it on unchanged, or attempt to repair it to increase

   the odds of successful delivery (or subsequent reply) began almost

   with the dawn of structured network mail and shows no signs of

   abating.  Advocates of rejection claim that attempted repairs are

   rarely completely adequate and that rejection of bad messages is the

   only way to get the offending software repaired.  Advocates of

   "repair" or "deliver no matter what" argue that users prefer that

   mail go through it if at all possible and that there are significant

   market pressures in that direction.  In practice, these market

   pressures may be more important to particular vendors than strict

   conformance to the standards, regardless of the preference of the

   actual developers.



   The problems associated with ill-formed messages were exacerbated by

   the introduction of the split-UA mail reading protocols [3, 26, 5,

   21].  These protocols have encouraged the use of SMTP as a posting

   protocol, and SMTP servers as relay systems for these client hosts

   (which are often only intermittently connected to the Internet).

   Historically, many of those client machines lacked some of the

   mechanisms and information assumed by SMTP (and indeed, by the mail

   format protocol [7]).  Some could not keep adequate track of time;

   others had no concept of time zones; still others could not identify

   their own names or addresses; and, of course, none could satisfy the

   assumptions that underlay RFC 822's conception of authenticated

   addresses.



   In response to these weak SMTP clients, many SMTP systems now

   complete messages that are delivered to them in incomplete or

   incorrect form.  This strategy is generally considered appropriate

   when the server can identify or authenticate the client, and there

   are prior agreements between them.  By contrast, there is at best

   great concern about fixes applied by a relay or delivery SMTP server

   that has little or no knowledge of the user or client machine.







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   The following changes to a message being processed MAY be applied

   when necessary by an originating SMTP server, or one used as the

   target of SMTP as an initial posting protocol:



   -  Addition of a message-id field when none appears



   -  Addition of a date, time or time zone when none appears



   -  Correction of addresses to proper FQDN format



   The less information the server has about the client, the less likely

   these changes are to be correct and the more caution and conservatism

   should be applied when considering whether or not to perform fixes

   and how.  These changes MUST NOT be applied by an SMTP server that

   provides an intermediate relay function.



   In all cases, properly-operating clients supplying correct

   information are preferred to corrections by the SMTP server.  In all

   cases, documentation of actions performed by the servers (in trace

   fields and/or header comments) is strongly encouraged.



7. Security Considerations



7.1 Mail Security and Spoofing



   SMTP mail is inherently insecure in that it is feasible for even

   fairly casual users to negotiate directly with receiving and relaying

   SMTP servers and create messages that will trick a naive recipient

   into believing that they came from somewhere else.  Constructing such

   a message so that the "spoofed" behavior cannot be detected by an

   expert is somewhat more difficult, but not sufficiently so as to be a

   deterrent to someone who is determined and knowledgeable.

   Consequently, as knowledge of Internet mail increases, so does the

   knowledge that SMTP mail inherently cannot be authenticated, or

   integrity checks provided, at the transport level.  Real mail

   security lies only in end-to-end methods involving the message

   bodies, such as those which use digital signatures (see [14] and,

   e.g., PGP [4] or S/MIME [31]).



   Various protocol extensions and configuration options that provide

   authentication at the transport level (e.g., from an SMTP client to

   an SMTP server) improve somewhat on the traditional situation

   described above.  However, unless they are accompanied by careful

   handoffs of responsibility in a carefully-designed trust environment,

   they remain inherently weaker than end-to-end mechanisms which use

   digitally signed messages rather than depending on the integrity of

   the transport system.









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   Efforts to make it more difficult for users to set envelope return

   path and header "From" fields to point to valid addresses other than

   their own are largely misguided: they frustrate legitimate

   applications in which mail is sent by one user on behalf of another

   or in which error (or normal) replies should be directed to a special

   address.  (Systems that provide convenient ways for users to alter

   these fields on a per-message basis should attempt to establish a

   primary and permanent mailbox address for the user so that Sender

   fields within the message data can be generated sensibly.)



   This specification does not further address the authentication issues

   associated with SMTP other than to advocate that useful functionality

   not be disabled in the hope of providing some small margin of

   protection against an ignorant user who is trying to fake mail.



7.2 "Blind" Copies



   Addresses that do not appear in the message headers may appear in the

   RCPT commands to an SMTP server for a number of reasons.  The two

   most common involve the use of a mailing address as a "list exploder"

   (a single address that resolves into multiple addresses) and the

   appearance of "blind copies".  Especially when more than one RCPT

   command is present, and in order to avoid defeating some of the

   purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy

   the full set of RCPT command arguments into the headers, either as

   part of trace headers or as informational or private-extension

   headers.  Since this rule is often violated in practice, and cannot

   be enforced, sending SMTP systems that are aware of "bcc" use MAY

   find it helpful to send each blind copy as a separate message

   transaction containing only a single RCPT command.



   There is no inherent relationship between either "reverse" (from

   MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP

   transaction ("envelope") and the addresses in the headers.  Receiving

   systems SHOULD NOT attempt to deduce such relationships and use them

   to alter the headers of the message for delivery.  The popular

   "Apparently-to" header is a violation of this principle as well as a

   common source of unintended information disclosure and SHOULD NOT be

   used.



7.3 VRFY, EXPN, and Security



   As discussed in section 3.5, individual sites may want to disable

   either or both of VRFY or EXPN for security reasons.  As a corollary

   to the above, implementations that permit this MUST NOT appear to

   have verified addresses that are not, in fact, verified.  If a site











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   disables these commands for security reasons, the SMTP server MUST

   return a 252 response, rather than a code that could be confused with

   successful or unsuccessful verification.



   Returning a 250 reply code with the address listed in the VRFY

   command after having checked it only for syntax violates this rule.

   Of course, an implementation that "supports" VRFY by always returning

   550 whether or not the address is valid is equally not in

   conformance.



   Within the last few years, the contents of mailing lists have become

   popular as an address information source for so-called "spammers."

   The use of EXPN to "harvest" addresses has increased as list

   administrators have installed protections against inappropriate uses

   of the lists themselves.  Implementations SHOULD still provide

   support for EXPN, but sites SHOULD carefully evaluate the tradeoffs.

   As authentication mechanisms are introduced into SMTP, some sites may

   choose to make EXPN available only to authenticated requestors.



7.4 Information Disclosure in Announcements



   There has been an ongoing debate about the tradeoffs between the

   debugging advantages of announcing server type and version (and,

   sometimes, even server domain name) in the greeting response or in

   response to the HELP command and the disadvantages of exposing

   information that might be useful in a potential hostile attack.  The

   utility of the debugging information is beyond doubt.  Those who

   argue for making it available point out that it is far better to

   actually secure an SMTP server rather than hope that trying to

   conceal known vulnerabilities by hiding the server's precise identity

   will provide more protection.  Sites are encouraged to evaluate the

   tradeoff with that issue in mind; implementations are strongly

   encouraged to minimally provide for making type and version

   information available in some way to other network hosts.



7.5 Information Disclosure in Trace Fields



   In some circumstances, such as when mail originates from within a LAN

   whose hosts are not directly on the public Internet, trace

   ("Received") fields produced in conformance with this specification

   may disclose host names and similar information that would not

   normally be available.  This ordinarily does not pose a problem, but

   sites with special concerns about name disclosure should be aware of

   it.  Also, the optional FOR clause should be supplied with caution or

   not at all when multiple recipients are involved lest it

   inadvertently disclose the identities of "blind copy" recipients to

   others.









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7.6 Information Disclosure in Message Forwarding



   As discussed in section 3.4, use of the 251 or 551 reply codes to

   identify the replacement address associated with a mailbox may

   inadvertently disclose sensitive information.  Sites that are

   concerned about those issues should ensure that they select and

   configure servers appropriately.



7.7 Scope of Operation of SMTP Servers



   It is a well-established principle that an SMTP server may refuse to

   accept mail for any operational or technical reason that makes sense

   to the site providing the server.  However, cooperation among sites

   and installations makes the Internet possible.  If sites take

   excessive advantage of the right to reject traffic, the ubiquity of

   email availability (one of the strengths of the Internet) will be

   threatened; considerable care should be taken and balance maintained

   if a site decides to be selective about the traffic it will accept

   and process.



   In recent years, use of the relay function through arbitrary sites

   has been used as part of hostile efforts to hide the actual origins

   of mail.  Some sites have decided to limit the use of the relay

   function to known or identifiable sources, and implementations SHOULD

   provide the capability to perform this type of filtering.  When mail

   is rejected for these or other policy reasons, a 550 code SHOULD be

   used in response to EHLO, MAIL, or RCPT as appropriate.



8. IANA Considerations



   IANA will maintain three registries in support of this specification.

   The first consists of SMTP service extensions with the associated

   keywords, and, as needed, parameters and verbs.  As specified in

   section 2.2.2, no entry may be made in this registry that starts in

   an "X".  Entries may be made only for service extensions (and

   associated keywords, parameters, or verbs) that are defined in

   standards-track or experimental RFCs specifically approved by the

   IESG for this purpose.



   The second registry consists of "tags" that identify forms of domain

   literals other than those for IPv4 addresses (specified in RFC 821

   and in this document) and IPv6 addresses (specified in this

   document).  Additional literal types require standardization before

   being used; none are anticipated at this time.



   The third, established by RFC 821 and renewed by this specification,

   is a registry of link and protocol identifiers to be used with the

   "via" and "with" subclauses of the time stamp ("Received: header")







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   described in section 4.4.  Link and protocol identifiers in addition

   to those specified in this document may be registered only by

   standardization or by way of an RFC-documented, IESG-approved,

   Experimental protocol extension.



9. References



   [1]  American National Standards Institute (formerly United States of

        America Standards Institute), X3.4, 1968, "USA Code for

        Information Interchange". ANSI X3.4-1968 has been replaced by

        newer versions with slight modifications, but the 1968 version

        remains definitive for the Internet.



   [2]  Braden, R., "Requirements for Internet hosts - application and

        support", STD 3, RFC 1123, October 1989.



   [3]  Butler, M., Chase, D., Goldberger, J., Postel, J. and J.

        Reynolds, "Post Office Protocol - version 2", RFC 937, February

        1985.



   [4]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP

        Message Format", RFC 2440, November 1998.



   [5]  Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC

        1176, August 1990.



   [6]  Crispin, M., "Internet Message Access Protocol - Version 4", RFC

        2060, December 1996.



   [7]  Crocker, D., "Standard for the Format of ARPA Internet Text

        Messages", RFC 822, August 1982.



   [8]  Crocker, D. and P. Overell, Eds., "Augmented BNF for Syntax

        Specifications: ABNF", RFC 2234, November 1997.



   [9]  De Winter, J., "SMTP Service Extension for Remote Message Queue

        Starting", RFC 1985, August 1996.



   [10] Fajman, R., "An Extensible Message Format for Message

        Disposition Notifications", RFC 2298, March 1998.



   [11] Freed, N, "Behavior of and Requirements for Internet Firewalls",

        RFC 2979, October 2000.



   [12] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

        Extensions (MIME) Part One: Format of Internet Message Bodies",

        RFC 2045, December 1996.









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   [13] Freed, N., "SMTP Service Extension for Command Pipelining", RFC

        2920, September 2000.



   [14] Galvin, J., Murphy, S., Crocker, S. and N. Freed, "Security

        Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",

        RFC 1847, October 1995.



   [15] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,

        December 1998.



   [16] Kille, S., "Mapping between X.400 and RFC822/MIME", RFC 2156,

        January 1998.



   [17] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing

        Architecture", RFC 2373, July 1998.



   [18] Klensin, J., Freed, N. and K. Moore, "SMTP Service Extension for

        Message Size Declaration", STD 10, RFC 1870, November 1995.



   [19] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,

        "SMTP Service Extensions", STD 10, RFC 1869, November 1995.



   [20] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,

        "SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July

        1994.



   [21] Lambert, M., "PCMAIL: A distributed mail system for personal

        computers", RFC 1056, July 1988.



   [22] Mockapetris, P., "Domain names - implementation and

        specification", STD 13, RFC 1035, November 1987.



        Mockapetris, P., "Domain names - concepts and facilities", STD

        13, RFC 1034, November 1987.



   [23] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part

        Three: Message Header Extensions for Non-ASCII Text", RFC 2047,

        December 1996.



   [24] Moore, K., "SMTP Service Extension for Delivery Status

        Notifications", RFC 1891, January 1996.



   [25] Moore, K., and G. Vaudreuil, "An Extensible Message Format for

        Delivery Status Notifications", RFC 1894, January 1996.



   [26] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD

        53, RFC 1939, May 1996.









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   [27] Partridge, C., "Mail routing and the domain system", RFC 974,

        January 1986.



   [28] Partridge, C., "Duplicate messages and SMTP", RFC 1047, February

        1988.



   [29] Postel, J., ed., "Transmission Control Protocol - DARPA Internet

        Program Protocol Specification", STD 7, RFC 793, September 1981.



   [30] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August

        1982.



   [31] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC

        2633, June 1999.



   [32] Resnick, P., Ed., "Internet Message Format", RFC 2822, April

        2001.



   [33] Vaudreuil, G., "SMTP Service Extensions for Transmission of

        Large and Binary MIME Messages", RFC 1830, August 1995.



   [34] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,

        January 1996.



10. Editor's Address



   John C. Klensin

   AT&T Laboratories

   99 Bedford St

   Boston, MA 02111 USA



   Phone: 617-574-3076

   EMail: klensin@research.att.com



11. Acknowledgments



   Many people worked long and hard on the many iterations of this

   document.  There was wide-ranging debate in the IETF DRUMS Working

   Group, both on its mailing list and in face to face discussions,

   about many technical issues and the role of a revised standard for

   Internet mail transport, and many contributors helped form the

   wording in this specification.  The hundreds of participants in the

   many discussions since RFC 821 was produced are too numerous to

   mention, but they all helped this document become what it is.















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APPENDICES



A. TCP Transport Service



   The TCP connection supports the transmission of 8-bit bytes.  The

   SMTP data is 7-bit ASCII characters.  Each character is transmitted

   as an 8-bit byte with the high-order bit cleared to zero.  Service

   extensions may modify this rule to permit transmission of full 8-bit

   data bytes as part of the message body, but not in SMTP commands or

   responses.



B. Generating SMTP Commands from RFC 822 Headers



   Some systems use RFC 822 headers (only) in a mail submission

   protocol, or otherwise generate SMTP commands from RFC 822 headers

   when such a message is handed to an MTA from a UA.  While the MTA-UA

   protocol is a private matter, not covered by any Internet Standard,

   there are problems with this approach.  For example, there have been

   repeated problems with proper handling of "bcc" copies and

   redistribution lists when information that conceptually belongs to a

   mail envelopes is not separated early in processing from header

   information (and kept separate).



   It is recommended that the UA provide its initial ("submission

   client") MTA with an envelope separate from the message itself.

   However, if the envelope is not supplied, SMTP commands SHOULD be

   generated as follows:



   1. Each recipient address from a TO, CC, or BCC header field SHOULD

      be copied to a RCPT command (generating multiple message copies if

      that is required for queuing or delivery).  This includes any

      addresses listed in a RFC 822 "group".  Any BCC fields SHOULD then

      be removed from the headers.  Once this process is completed, the

      remaining headers SHOULD be checked to verify that at least one

      To:, Cc:, or Bcc: header remains.  If none do, then a bcc: header

      with no additional information SHOULD be inserted as specified in

      [32].



   2. The return address in the MAIL command SHOULD, if possible, be

      derived from the system's identity for the submitting (local)

      user, and the "From:" header field otherwise.  If there is a

      system identity available, it SHOULD also be copied to the Sender

      header field if it is different from the address in the From

      header field.  (Any Sender field that was already there SHOULD be

      removed.)  Systems may provide a way for submitters to override

      the envelope return address, but may want to restrict its use to

      privileged users.  This will not prevent mail forgery, but may

      lessen its incidence; see section 7.1.







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   When an MTA is being used in this way, it bears responsibility for

   ensuring that the message being transmitted is valid.  The mechanisms

   for checking that validity, and for handling (or returning) messages

   that are not valid at the time of arrival, are part of the MUA-MTA

   interface and not covered by this specification.



   A submission protocol based on Standard RFC 822 information alone

   MUST NOT be used to gateway a message from a foreign (non-SMTP) mail

   system into an SMTP environment.  Additional information to construct

   an envelope must come from some source in the other environment,

   whether supplemental headers or the foreign system's envelope.



   Attempts to gateway messages using only their header "to" and "cc"

   fields have repeatedly caused mail loops and other behavior adverse

   to the proper functioning of the Internet mail environment.  These

   problems have been especially common when the message originates from

   an Internet mailing list and is distributed into the foreign

   environment using envelope information.  When these messages are then

   processed by a header-only remailer, loops back to the Internet

   environment (and the mailing list) are almost inevitable.



C. Source Routes



   Historically, the <reverse-path> was a reverse source routing list of

   hosts and a source mailbox.  The first host in the <reverse-path>

   SHOULD be the host sending the MAIL command.  Similarly, the

   <forward-path> may be a source routing lists of hosts and a

   destination mailbox.  However, in general, the <forward-path> SHOULD

   contain only a mailbox and domain name, relying on the domain name

   system to supply routing information if required.  The use of source

   routes is deprecated; while servers MUST be prepared to receive and

   handle them as discussed in section 3.3 and F.2, clients SHOULD NOT

   transmit them and this section was included only to provide context.



   For relay purposes, the forward-path may be a source route of the

   form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST BE fully-

   qualified domain names.  This form is used to emphasize the

   distinction between an address and a route.  The mailbox is an

   absolute address, and the route is information about how to get

   there.  The two concepts should not be confused.



   If source routes are used, RFC 821 and the text below should be

   consulted for the mechanisms for constructing and updating the

   forward- and reverse-paths.















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   The SMTP server transforms the command arguments by moving its own

   identifier (its domain name or that of any domain for which it is

   acting as a mail exchanger), if it appears, from the forward-path to

   the beginning of the reverse-path.



   Notice that the forward-path and reverse-path appear in the SMTP

   commands and replies, but not necessarily in the message.  That is,

   there is no need for these paths and especially this syntax to appear

   in the "To:" , "From:", "CC:", etc. fields of the message header.

   Conversely, SMTP servers MUST NOT derive final message delivery

   information from message header fields.



   When the list of hosts is present, it is a "reverse" source route and

   indicates that the mail was relayed through each host on the list

   (the first host in the list was the most recent relay).  This list is

   used as a source route to return non-delivery notices to the sender.

   As each relay host adds itself to the beginning of the list, it MUST

   use its name as known in the transport environment to which it is

   relaying the mail rather than that of the transport environment from

   which the mail came (if they are different).



D. Scenarios



   This section presents complete scenarios of several types of SMTP

   sessions.  In the examples, "C:" indicates what is said by the SMTP

   client, and "S:" indicates what is said by the SMTP server.



D.1 A Typical SMTP Transaction Scenario



   This SMTP example shows mail sent by Smith at host bar.com, to Jones,

   Green, and Brown at host foo.com.  Here we assume that host bar.com

   contacts host foo.com directly.  The mail is accepted for Jones and

   Brown.  Green does not have a mailbox at host foo.com.



      S: 220 foo.com Simple Mail Transfer Service Ready

      C: EHLO bar.com

      S: 250-foo.com greets bar.com

      S: 250-8BITMIME

      S: 250-SIZE

      S: 250-DSN

      S: 250 HELP

      C: MAIL FROM:<Smith@bar.com>

      S: 250 OK

      C: RCPT TO:<Jones@foo.com>

      S: 250 OK

      C: RCPT TO:<Green@foo.com>

      S: 550 No such user here

      C: RCPT TO:<Brown@foo.com>







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      S: 250 OK

      C: DATA

      S: 354 Start mail input; end with <CRLF>.<CRLF>

      C: Blah blah blah...

      C: ...etc. etc. etc.

      C: .

      S: 250 OK

      C: QUIT

      S: 221 foo.com Service closing transmission channel



D.2 Aborted SMTP Transaction Scenario



      S: 220 foo.com Simple Mail Transfer Service Ready

      C: EHLO bar.com

      S: 250-foo.com greets bar.com

      S: 250-8BITMIME

      S: 250-SIZE

      S: 250-DSN

      S: 250 HELP

      C: MAIL FROM:<Smith@bar.com>

      S: 250 OK

      C: RCPT TO:<Jones@foo.com>

      S: 250 OK

      C: RCPT TO:<Green@foo.com>

      S: 550 No such user here

      C: RSET

      S: 250 OK

      C: QUIT

      S: 221 foo.com Service closing transmission channel



D.3 Relayed Mail Scenario



   Step 1  --  Source Host to Relay Host



      S: 220 foo.com Simple Mail Transfer Service Ready

      C: EHLO bar.com

      S: 250-foo.com greets bar.com

      S: 250-8BITMIME

      S: 250-SIZE

      S: 250-DSN

      S: 250 HELP

      C: MAIL FROM:<JQP@bar.com>

      S: 250 OK

      C: RCPT TO:<@foo.com:Jones@XYZ.COM>

      S: 250 OK

      C: DATA

      S: 354 Start mail input; end with <CRLF>.<CRLF>

      C: Date: Thu, 21 May 1998 05:33:29 -0700







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      C: From: John Q. Public <JQP@bar.com>

      C: Subject:  The Next Meeting of the Board

      C: To: Jones@xyz.com

      C:

      C: Bill:

      C: The next meeting of the board of directors will be

      C: on Tuesday.

      C:                         John.

      C: .

      S: 250 OK

      C: QUIT

      S: 221 foo.com Service closing transmission channel



   Step 2  --  Relay Host to Destination Host



      S: 220 xyz.com Simple Mail Transfer Service Ready

      C: EHLO foo.com

      S: 250 xyz.com is on the air

      C: MAIL FROM:<@foo.com:JQP@bar.com>

      S: 250 OK

      C: RCPT TO:<Jones@XYZ.COM>

      S: 250 OK

      C: DATA

      S: 354 Start mail input; end with <CRLF>.<CRLF>

      C: Received: from bar.com by foo.com ; Thu, 21 May 1998

      C:     05:33:29 -0700

      C: Date: Thu, 21 May 1998 05:33:22 -0700

      C: From: John Q. Public <JQP@bar.com>

      C: Subject:  The Next Meeting of the Board

      C: To: Jones@xyz.com

      C:

      C: Bill:

      C: The next meeting of the board of directors will be

      C: on Tuesday.

      C:                         John.

      C: .

      S: 250 OK

      C: QUIT

      S: 221 foo.com Service closing transmission channel



D.4 Verifying and Sending Scenario



      S: 220 foo.com Simple Mail Transfer Service Ready

      C: EHLO bar.com

      S: 250-foo.com greets bar.com

      S: 250-8BITMIME

      S: 250-SIZE

      S: 250-DSN







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      S: 250-VRFY

      S: 250 HELP

      C: VRFY Crispin

      S: 250 Mark Crispin <Admin.MRC@foo.com>

      C: SEND FROM:<EAK@bar.com>

      S: 250 OK

      C: RCPT TO:<Admin.MRC@foo.com>

      S: 250 OK

      C: DATA

      S: 354 Start mail input; end with <CRLF>.<CRLF>

      C: Blah blah blah...

      C: ...etc. etc. etc.

      C: .

      S: 250 OK

      C: QUIT

      S: 221 foo.com Service closing transmission channel



E. Other Gateway Issues



   In general, gateways between the Internet and other mail systems

   SHOULD attempt to preserve any layering semantics across the

   boundaries between the two mail systems involved.  Gateway-

   translation approaches that attempt to take shortcuts by mapping,

   (such as envelope information from one system to the message headers

   or body of another) have generally proven to be inadequate in

   important ways.  Systems translating between environments that do not

   support both envelopes and headers and Internet mail must be written

   with the understanding that some information loss is almost

   inevitable.



F. Deprecated Features of RFC 821



   A few features of RFC 821 have proven to be problematic and SHOULD

   NOT be used in Internet mail.



F.1 TURN



   This command, described in RFC 821, raises important security issues

   since, in the absence of strong authentication of the host requesting

   that the client and server switch roles, it can easily be used to

   divert mail from its correct destination.  Its use is deprecated;

   SMTP systems SHOULD NOT use it unless the server can authenticate the

   client.

















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F.2 Source Routing



   RFC 821 utilized the concept of explicit source routing to get mail

   from one host to another via a series of relays.  The requirement to

   utilize source routes in regular mail traffic was eliminated by the

   introduction of the domain name system "MX" record and the last

   significant justification for them was eliminated by the

   introduction, in RFC 1123, of a clear requirement that addresses

   following an "@" must all be fully-qualified domain names.

   Consequently, the only remaining justifications for the use of source

   routes are support for very old SMTP clients or MUAs and in mail

   system debugging.  They can, however, still be useful in the latter

   circumstance and for routing mail around serious, but temporary,

   problems such as problems with the relevant DNS records.



   SMTP servers MUST continue to accept source route syntax as specified

   in the main body of this document and in RFC 1123.  They MAY, if

   necessary, ignore the routes and utilize only the target domain in

   the address.  If they do utilize the source route, the message MUST

   be sent to the first domain shown in the address.  In particular, a

   server MUST NOT guess at shortcuts within the source route.



   Clients SHOULD NOT utilize explicit source routing except under

   unusual circumstances, such as debugging or potentially relaying

   around firewall or mail system configuration errors.



F.3 HELO



   As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to

   HELO when the server will accept the former.  Servers must continue

   to accept and process HELO in order to support older clients.



F.4 #-literals



   RFC 821 provided for specifying an Internet address as a decimal

   integer host number prefixed by a pound sign, "#".  In practice, that

   form has been obsolete since the introduction of TCP/IP.  It is

   deprecated and MUST NOT be used.



F.5 Dates and Years



   When dates are inserted into messages by SMTP clients or servers

   (e.g., in trace fields), four-digit years MUST BE used.  Two-digit

   years are deprecated; three-digit years were never permitted in the

   Internet mail system.













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F.6 Sending versus Mailing



   In addition to specifying a mechanism for delivering messages to

   user's mailboxes, RFC 821 provided additional, optional, commands to

   deliver messages directly to the user's terminal screen.  These

   commands (SEND, SAML, SOML) were rarely implemented, and changes in

   workstation technology and the introduction of other protocols may

   have rendered them obsolete even where they are implemented.



   Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers

   MAY implement them.  If they are implemented by servers, the

   implementation model specified in RFC 821 MUST be used and the

   command names MUST be published in the response to the EHLO command.













































































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Full Copyright Statement



   Copyright (C) The Internet Society (2001).  All Rights Reserved.



   This document and translations of it may be copied and furnished to

   others, and derivative works that comment on or otherwise explain it

   or assist in its implementation may be prepared, copied, published

   and distributed, in whole or in part, without restriction of any

   kind, provided that the above copyright notice and this paragraph are

   included on all such copies and derivative works.  However, this

   document itself may not be modified in any way, such as by removing

   the copyright notice or references to the Internet Society or other

   Internet organizations, except as needed for the purpose of

   developing Internet standards in which case the procedures for

   copyrights defined in the Internet Standards process must be

   followed, or as required to translate it into languages other than

   English.



   The limited permissions granted above are perpetual and will not be

   revoked by the Internet Society or its successors or assigns.



   This document and the information contained herein is provided on an

   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

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Acknowledgement



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Klensin                     Standards Track                    [Page 79]



