RFC 2821 - Simple Mail Transfer Protocol
- Describes the widely implemented Simple Mail Transfer Protocol (SMTP) designed to transfer mail reliably and efficiently and relay mail across transport service environments. Obsoletes RFC 821, 974, 1869.
- http://tools.ietf.org/html/rfc2821

RFC 947 - Mail Routing and the Domain System
- Describes how mail systems on the Internet are expected to route messages based on information from the domain system.
- http://tools.ietf.org/html/rfc974

RFC 1566 - Mail Monitoring MIB
- Defines a portion of the Management Information Base (MIB) for use with network management protocols, and extends the basic Network Services Monitoring MIB to allow monitoring of Message Transfer Agents (MTAs).
- http://tools.ietf.org/html/rfc1566

RFC 2049 - Multipurpose Internet Mail Extensions (MIME) - Part 5
- Describes MIME conformance criteria as well as providing some illustrative examples of MIME message formats.
- http://tools.ietf.org/html/rfc2049

RFC 2048 - Multipurpose Internet Mail Extensions (MIME) - Part 4
- Specifies various IANA registration procedures for MIME media types, MIME external body access types, and MIME content-transfer-encodings.
- http://tools.ietf.org/html/rfc2048

RFC 1123 - Requirements for Internet Hosts - Application and Support
- Defines and discusses the requirements for Internet host software and covers the application and support protocols.
- http://tools.ietf.org/html/rfc1123

RFC 2045 - Multipurpose Internet Mail Extensions (MIME) - Part 1
- Specifies the format of Internet message bodies and the various headers used to describe the structure of MIME messages.
- http://tools.ietf.org/html/rfc2045

RFC 1893 - Enhanced Mail System Status Codes
- Describes the Enhanced Mail System Status Codes, a set of status codes for reporting mail system errors and delivery status notifications.
- http://tools.ietf.org/html/rfc1893

RFC 2060 - Internet Message Access Protocol (IMAP) - Version 4 Rev 1
- Describes the Internet Message Access Protocol (IMAP), which allows a client to access and manipulate electronic mail messages on a server, and permits manipulation of remote message folders.
- http://tools.ietf.org/html/rfc2060

RFC 1869 - SMTP Service Extensions (ESMTP)
- Defines a framework for extending the SMTP service by defining a means whereby a server can inform a client as to the service extensions it supports.
- http://tools.ietf.org/html/rfc1869

RFC 822 - Standard for ARPA Internet Text Messages
- Specifies the syntax for Internet mail text messages.
- http://tools.ietf.org/html/rfc822

RFC 2047 - Multipurpose Internet Mail Extensions (MIME) - Part 3
- Describes extensions to RFC 822 to allow non-US-ASCII text data in Internet mail header fields.
- http://tools.ietf.org/html/rfc2047

RFC 2046 - Multipurpose Internet Mail Extensions (MIME) - Part 2
- Defines the general structure of the MIME media typing system and defines an initial set of media types.
- http://tools.ietf.org/html/rfc2046

RFC 2033 - Local Mail Transfer Protocol (LMTP)
- Describes the Local Mail Transfer Protocol (LMTP), for transporting mail into a system where a mail receiver does not manage a queue.
- http://tools.ietf.org/html/rfc2033