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Table Of Contents  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

Previous Topic/Section
TCP/IP Electronic Mail RFC 822 Standard Message Format Overview, Structure and General Formatting Rules
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TCP/IP Electronic Mail RFC 822 Standard Message Format Processing and Interpretation
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TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups
(Page 1 of 3)

The RFC 822 message format describes the structure and content of TCP/IP e-mail messages. The structure is intentionally designed to be very simple and easy to both create and understand. Each message begins with a set of headers that describe the message and its contents. An empty line marks the end of the headers, and then the message body follows.

The message body contains the actual text that the sender is trying to communicate to the recipient(s), while the message header contains various types of information that serve various purposes. The headers help control how the message is processed, by specifying who the recipients are, describing the contents of the message, and providing information to a recipient of a message about processing done on the message as it was delivered.

Header Field Structure

Each header field follows the simple text structure we saw in the preceding topic:

<header name>: <header value>

The <header name> is of course the name of the header, and the <header value> is the value associated with that header, which depends on the header type. Like all RFC 822 lines, headers must be no more than 998 characters long and are recommended to be no more than 78 characters in length, for easier readability. The RFC 822 and 2822 standards support a special syntax for allowing headers to be “folded” onto multiple lines if they are very lengthy. This is done by simply continuing a header value onto a new line, which must begin with at least one “white space” character, such as a space or <Tab> character, like this:

<header name>: <header value part 1>
<white space> <header value part 2>
<white space> <header value part 3>

The <Tab> character is most often used for this purpose. So, for example, if we wanted to specify a large number of recipients for a message, we could do it as follows:

To: person1@domain1.org, person2@domain2.com,
person3@domain3.net, person4@domain4.edu

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TCP/IP Electronic Mail RFC 822 Standard Message Format Overview, Structure and General Formatting Rules
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Version 3.0 - Version Date: September 20, 2005

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