One of the most astute observations I have read about internetworking applications is that their usefulness is proportional to the number of people who use them. TCP/IP e-mail is a great example: it is a powerful communication method in large part because almost everyone with a computer today participates in the system. The more people who sign on to use e-mail, the more powerful it becomes.

The creators of TCP/IP email realized that people who use the system would employ many different types of hardware and software. To ensure that everyone was able to understand all e-mail messages, regardless of who sent them, they specified a common message format for electronic mail messages. This format doesn't have an official fancy name; it is simply known by the name of the standard that defines it: the RFC 822 message format.

In this section I describe the RFC 822 message format, which forms the basis for electronic mail message transfer in TCP/IP. I begin with an overview of the format and the general structure of messages, and some of the overall rules used to format RFC 822 messages. I then describe the many headers used in RFC 822 e-mail messages, and how they are arranged into groups. I conclude with a brief look at how RFC 822 messages are processed and their contents interpreted.