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The primary protocol for delivering electronic mail is the Simple Mail Transfer Protocol (SMTP). For this reason, the message format used for TCP/IP e-mail could be considered SMTP's “protocol message format”, not unlike the special message formats discussed for other protocols, such as IP and TCP. However, the TCP/IP e-mail message format is used not only by SMTP but by all protocols and applications that deal with electronic mail. This includes the mail access protocols POP3 and IMAP, as well as others. It was also intended to potentially be usable by other non-TCP/IP mail delivery protocols.
Perhaps for this reason, the TCP/IP e-mail format was not specified as part of the SMTP itself, RFC 821, but rather in a companion document: RFC 822. Both were published in 1982. No official fancy name was given to this message format, and as a result the format became known by the name of the standard itself: the RFC 822 message format.
The history of the message format used in TCP/IP goes back long before 1982, of course. It was originally defined as the format for passing text messages on the Internet’s precursor, the ARPAnet, in the early 1970s. Over time the format was refined several times, leading to the publication, in 1977, of the important e-mail standard RFC 733 (Standard for the Format of ARPA Network Text Messages). RFC 822 later streamlined the contents of RFC 733, removing some of the features described in the earlier standard that failed to gain acceptance, and simplifying the specification.
In 2001, both SMTP and the RFC 822 message format were revised; SMTP is now described in RFC 2821 and the message format in RFC 2822. This newer standard makes relatively small changes to the RFC 822 message format to reflect modern use of TCP/IP e-mail. Even though RFC 2822 is the current standard, the original name is still the one most commonly used. I will respect that convention in this discussion, describing the message format based on RFC 2822 while still calling it the “RFC 822” message format.
The RFC 822 format describes the form, structure and content of TCP/IP electronic mail messages. It is, as I said, analogous to the message formats used for other protocols in TCP/IP. Like those other formats, the RFC 822 format can be logically divided into two main sections: the message header, which contains important control and descriptive information, and the message body or payload, which carries the data.