The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME                           9  TCP/IP Electronic Mail Standard Message Format: RFC 822

TCP/IP Electronic Mail Standard Message Format: RFC 822 1 2 3 TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups

General Structure

The RFC 822 message always starts with a set of header fields as described above; the next topic describes them in more detail. After all the headers, an empty line must occur. This consists simply of the characters “CRLF” by themselves, immediately following the “CRLF” at the end of the final header field line. Seeing two “CRLF” character pairs in sequence tells the device reading the message that the end of the headers have been reached. All the remaining lines are considered the body of the message. Like the header lines, body lines are comprised of ASCII text and must be no more than 998 characters, with 78 characters or less recommended (for easier reading on standard 80-character terminal displays).

Since both the header and body of e-mail messages are simply ASCII text, this means the entire message is just a text file. This makes these messages very readable, as I said above, and also quite easy to create. One can use a simple text editor to create a complete electronic mail message, including headers, and can read it with a simple text display utility. This contributes to e-mail's universal appeal.

The drawback is that the decision to make messages entirely ASCII means there is no native support in RFC 822 messages for anything that requires more complex structuring, or that cannot be expressed using the small number of ASCII characters. One cannot express pictures, or binary files, spreadsheets, sound clips and so forth directly using ASCII. Also, the use of ASCII makes RFC 822 well-suited to expressing messages in English, but not in many other languages, which use characters that ASCII cannot represent. All of these limitations eventually prompted the creation of the enhanced MIME message format.

TCP/IP Electronic Mail Standard Message Format: RFC 822 1 2 3 TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups