POP3

RFC 1939 - Post Office Protocol (version 3)
RFC 2449 - POP3 Extension Mechanism
RFC 1734 - POP3 AUTHentication command

POP3 and IMAP Specifications

RFC 2195 - IMAP/POP AUTHorize Extension for Simple Challenge/Response
RFC 2595 - Using TLS with IMAP, POP3 and ACAP

SMTP specifications

RFC 2821 - Simple Mail Transfer Protocol
RFC 821 - Simple Mail Transfer Protocol (obsolete)
RFC 822 - Format of ARPA Internet text messages
RFC 974 - Mail routing and the domain system
RFC 3501 - Internet message access protocol (version 4rev1)
RFC 3848 - ESMTP and LMTP Transmission Types Registration

SMTP service extensions

RFC 2821 - Simple Mail Transfer Protocol
RFC 1869 - SMTP Service Extensions
RFC 2554 - SMTP Service Extension for Authentication
RFC 1830 - SMTP Service Extensions for Transmission of Large and Binary MIME Messages
RFC 2920 - SMTP Service Extension for Command Pipelining
RFC 1652 - SMTP Service Extension for 8bit-MIME transport
RFC 1870 - SMTP Service Extension for Message Size Declaration

IMAP specifications

RFC 3501 - INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1
RFC 2342 - IMAP4 Namespace
RFC 2180 - IMAP4 Multi-Accessed Mailbox Practice
RFC 2683  - IMAP4 Implementation Recommendations
RFC 2087 - IMAP4 QUOTA extension
RFC 2359 - IMAP4 UIDPLUS extension
RFC 2088 - IMAP4 non-synchronizing literals
RFC 2177 - IMAP4 IDLE command
RFC 3502 - Internet Message Access Protocol (IMAP) - MULTIAPPEND Extension
RFC 3348 - The Internet Message Action Protocol (IMAP4) Child Mailbox Extension
RFC 4314 - IMAP4 Access Control List (ACL) Extension

HTTP specifications:

RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1
RFC 2965 - HTTP State Management Mechanism
RFC 2396 - Uniform Resource Identifiers (URI): Generic Syntax

DNS specifications

RFC 1034 - Domain names, Concepts and Facilities
RFC 1035 - Domain names, Implementation and Specification

Sieve extensions implemented in AXIGEN

RFC 3028 - Sieve: A Mail Filtering Language (Extensions defined in the base RFC: fileinto, reject, envelope)
RFC 3894 - Sieve Extension: Copying without Side Effects
RFC 3431 - Sieve Extension: Relational Tests; Comparator extension: i;numeric-comparator
RFC 3598 - Sieve Email Filtering -- Subaddress Extension

Generic RFCs

RFC 2822 - Internet message format
RFC 2045 - MIME Part One: Format of Internet Message Bodies 
RFC 2046 - MIME Part Two: Media Types 
RFC 2047 - MIME Part Three: Message Header Extensions for Non-ASCII Text

Mailing Lists

RFC 2919 - List-Id: A Structured Field and Namespace for the Identification of Mailing Lists
RFC 2369 - The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields

FTP

RFC 959 - FILE TRANSFER PROTOCOL (FTP)

Groupware

RFC 2445 - Internet Calendaring and Scheduling Core Object Specification (iCalendar)
RFC 2446 - iCalendar Transport-Independent Interoperability Protocol (iTIP) Scheduling Events, BusyTime, To-dos and Journal Entries
RFC 2447 - iCalendar Message-Based Interoperability Protocol (iMIP)
RFC 3283  - Guide to Internet Calendaring
RFC 2426 - vCard MIME Directory Profile

SNMP

RFC 1157 - A Simple Network Management Protocol (SNMP)
RFC 3416 - Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)
RFC 1213 - Management Information Base for Network Management of TCP/IP-based internets: MIB-II
RFC 3418 - Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)