Interesting RFC:s

The Tao of IETF: A Novice's Guide to the Internet Engineering task Force

Email
Email, SMTP and message formats (Wikipedia)

The Simple Mail Transfer Protocol
SMTP (Wikipedia)
SMTP (rfc 2821)

Internet Message Format
Internet Message Format (rfc 2822)

MIME
MIME (Wikipedia)
MIME Part One Format of Internet Message Bodies (rfc 2045)
MIME Part Two Media Types (rfc 2046)
MIME Part Three Message Header Extensions for Non-ASCII Text (rfc 2047)
Security Multiparts for MIME (rfc 1847)

S/MIME
S/MIME (Wikipedia)
S/MIME Version 3.1 (rfc 3851)

PKCS#7
PKCS#7 Version 1.5 (rfc 2315)

Cryptographic Message Syntax, CMS
CMS (Wikipedia)
CMS (rfc 3852)
CMS Algorithms (rfc 3370)
Compressed Data Content for CMS (rfc 3274)

Public Key Infrastructure, PKI
Public Key Infrastructure (X.509 v3) (rfc 3280)
Public Key Infrastructure Working Group (PKIX)
Peter Gutmann's X.509 Style Guide

TLS
TLS (Wikipedia)
TLS Version 1.1 (rfc 4346)

HTTP
HTTP (Wikipedia)
HTTP/1.1 (rfc 2616)

AS2
AS2 (Wikipedia)
AS2 (data interchange)
AS2 (rfc 4130)

MDN
Message Disposition Notification (rfc 3798)

OFTP
OFTP (Wikipedia)
OFTP 2.0 (data interchange)
OFTP 2.0 (rfc 5024)

List of RFC (Wikipedia)