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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 RFC 822 Standard Message Format Overview, Structure and General Formatting Rules 1 2 3 TCP/IP Electronic Mail RFC 822 Standard Message Format Processing and Interpretation

Header Field Groups

The RFC 822 message format specifies many types of headers that can be included in e-mail messages. A small number of headers are mandatory, meaning they must be included in all messages. Some are not mandatory but are usually present, because they are fundamental to describing the message. Others are optional and are included only when needed.

To help organize the many headers, the RFC 2822 standard categorizes them into header field groups (as did RFC 822, though the groups are a little different in the older standard):

• Origination Date Field: Specifies the date and time that the message was made ready for delivery; see below for details. (This field is in its own group for reasons that are unclear to me; perhaps just because it is important.)
  
  
• Originator Fields: Contain information about the sender of the message.
  
  
• Destination Address Fields: Specify the recipient(s) of the message, which may be in one of three different recipient classes.
  
  
• Identification Fields: Contain information to help identify the message.
  
  
• Informational Fields: Contain optional information to help make more clear to the recipient what the message is about.
  
  
• Resent Fields: Used to preserve the original originator, destination and other fields when a message is resent.
  
  
• Trace Fields: Used to show the path taken by mail as it was transported.

In addition, the format allows other, user-defined fields to be specified, as long as they correspond to the standard “<header name>: <header value>” syntax. This can be used to provide additional information of various sorts. For example, sometimes the e-mail client software will include a header line indicating the name and version of the software used to compose and send the message. We'll also see that MIME uses new header lines to encode information about MIME messages.

TCP/IP Electronic Mail RFC 822 Standard Message Format Overview, Structure and General Formatting Rules 1 2 3 TCP/IP Electronic Mail RFC 822 Standard Message Format Processing and Interpretation