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 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 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 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 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

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

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