SMTP Overview

Simple Mail Transfer Protocol (SMTP), documented in RFC 821, is Internet's standard host-to-host mail transport protocol and traditionally operates over TCP, port 25. In other words, a UNIX user can type telnet hostname 25 and connect with an SMTP server, if one is present.

SMTP uses a style of asymmetric request-response protocol popular in the early 1980s, and still seen occasionally, most often in mail protocols. The protocol is designed to be equally useful to either a computer or a human, though not too forgiving of the human. From the server's viewpoint, a clear set of commands is provided and well-documented in the RFC. For the human, all the commands are clearly terminated by newlines and a HELP command lists all of them. From the sender's viewpoint, the command replies always take the form of text lines, each starting with a three-digit code identifying the result of the operation, a continuation character to indicate another lines following, and then arbitrary text information designed to be informative to a human.

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