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.

RFC 4871: (Obsoleted by 6376)
---------
DomainKeys Identified Mail (DKIM) Signatures

RFC 5672: (Obsoleted by 6376)
---------
DomainKeys Identified Mail (DKIM) Signatures -- Update

RFC 6376:
---------
DomainKeys Identified Mail (DKIM) Signatures
