I have emphasized in my overall description of TCP/IP electronic mail that communication using e-mail requires the interaction of various protocols and elements. One mistake that some people make is to equate the method used for delivering e-mail with the entire system. This is, however, an understandable mistake: just as the postal service is only a part of the whole system of mailing a letter; it is nonetheless a very big part. Likewise, the delivery of e-mail from sender to recipient is arguably the most important part of e-mail as a whole. In modern TCP/IP, this task is the responsibility of the Simple Mail Transfer Protocol (SMTP).

In this section, I describe in detail the operation of the Simple Mail Transfer Protocol (SMTP). I begin with an overview and history of SMTP and a discussion of the standards that define it. I then examine the way that SMTP client/server communication and message transport works. I explain the way that SMTP servers establish connections and transaction sessions, and then the process by which mail is transferred from one to another. I describe some of the special features implemented in SMTP, and discuss SMTP security issues as well. I conclude with a reference summary of SMTP commands and replies.