RFC 1854:

---------

PIPELINING extension



RFC 2222:

---------

SASL



RFC 4505:

---------

ANYNONYMOUS SASL



RFC 4616:

---------

PLAIN SASL



RFC 2487:

---------

STARTTLS extension



RFC 2554 & 4954:

----------------

AUTH extension



RFC 2821:

---------

SMTP protocol



RFC 2822:

---------

General message structure (focusing on important headers)



RFC 2045:

---------

Quoted Printable Encoding

Base 64 Encoding

Detailed message structure



RFC 2046:

---------

Media types (for subparts)



RFC 2047:

---------

Header Encoding



RFC 2183:

---------

The Content-Disposition header



RFC 2231:

---------

Encoded Text header/attribute extensions



RFC 2234:

---------

ABNF definitions



RFC 3676:

---------

Flowed formatting/delsp parameters



RFC 2015:

---------

Original Mime Security with OpenPGP



RFC 2440:

---------

Original OpenPGP message format



RFC 3156:

---------

Current (30/01/2009) Mime Security with OpenPGP



RFC 4880:

---------

Current (30/01/2009) OpenPGP Message format revision (fun, it's exactly original *2 --sorry)



RFC 5751:

--------

Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2



RFC 4870:

---------

DomainKeys (historic), but still used by a few people.



RFX 4871:

---------

DKIM



