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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)

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TCP/IP Electronic Mail Aliases / Address Books, Multiple Recipient Addressing and Electronic Mailing Lists
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TCP/IP Electronic Mail Standard Message Format: RFC 822
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TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME

The advantages of using computers for communication are obvious, but there are also some limitations that are imposed by the use of computer technology. When I compare electronic mail to regular mail, I always point out that e-mail is much faster and more flexible in how it can be delivered, and this is true. An electronic mail message can reach its destination in seconds, where a conventional letter would take days.

But one drawback computers have is that they are not very adaptable in figuring out how to understand messages. Consider that anyone can put any type of letter, memorandum or other communication in an envelope and send it to you, and assuming you know the language it is written in, you can open the envelope and understand it. You can automatically figure out how to deal with the date being in an unusual place in the letter, or your name appearing at the top compared to the bottom, or the body of the message being structured in different ways. You can read notes that are typed or hand-written; in pen, pencil or crayon; as long as the letters are decipherable, you can understand what is being said.

Computers are not very good at this at all. And it is for that reason that e-mail systems must rely on standard message formats to ensure that all messages have the same form and structure. This then makes it possible for all devices in the electronic mail system are able to read and understand each others' messages, to enable TCP/IP e-mail to work on many different types of computers.

In this section, I describe the two formats used for TCP/IP electronic mail messages, in two subsections. The first describes the main TCP/IP electronic mail standard, which is called the RFC 822 format after the standard that defines it. The second describes the Multipurpose Internet Mail Extensions (MIME) standard, which greatly expands the ability of electronic mail to support the communication of different types of information, by defining methods of encoding various media and non-English language text into the standard RFC 822 format.

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