It is common for human beings to create systems that are reminiscent of ones to which they are already accustomed. We are all accustomed to using the regular mail system to send letters and other documents from our location to recipients anywhere that the postal system serves. Naturally, one of the first applications of internetworks was to create an electronic version of this conventional mail system that would allow messages to be sent in a similar manner, but more quickly and easily. Over the course of many years, an electronic mail system for TCP/IP was created and refined. It is now the most widely used means of electronic messaging in the world.

In this section I describe TCP/IP electronic mail in detail, in five sections that discuss electronic mail concepts and the various components and protocols that comprise the overall TCP/IP e-mail system. The first subsection provides an overview of TCP/IP electronic mail and discusses the way that it is used and the different protocols and methods that comprise the system. The second discusses how e-mail messages are addressed, and the third covers standard and special formats for e-mail messages. The fourth and fifth subsections describe the TCP/IP protocols that implement electronic mail functionality. This includes an examination of the Simple Mail Transfer Protocol (SMTP), which is responsible for the delivery of e-mail, and several protocols and methods used for mailbox access and mail retrieval, including POP3 and IMAP.