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Email Servers

SMTP's strength comes primarily from its simplicity. Experience with many protocols has shown that: protocols with few options tend towards ubiquity, whilst protocols with many options tend towards obscurity.

- Marshall Rose; SMTP Service Extensions; RFC 1425; February 1993.

Email servers exchange email with the Simple Mail Transfer Protocol (SMTP). Each Internet domain has a corresponding email server. When you send email, your client application first sends it to your email server, which then contacts the addressee's email server and carries out a conversation over the Internet according to the rules defined by SMTP. Your email server asks the other email server if the user name is valid, and, if it is, transfers the email, where the receiving email server stores it until the addressee logs on and downloads it.

By far the most common SMTP server in use is the venerable sendmail system, first distributed for free with the Unix operating system.

The list of commands that can be exchanged during an SMTP session between two email servers are listed below. The first command of an SMTP conversation must be the HELO command. A mail transaction is begun with the MAIL command. The last command in a session must be the QUIT command.

Command	Expanded Command
DATA	DATA
EXPN	EXPAND
HELO	HELLO
HELP	HELP
MAIL	MAIL
NOOP	NOOP
QUIT	QUIT
RCPT	RECIPIENT
RSET	RESET
VRFY	VERIFY

Resources. The following resources provide more information on email servers:

SMTP was first described in detail in RFC 821 in August, 1982 by the Internet pioneer Jonathan Postel. It has since been updated by RFC 2821.

The following resources provide options for running your own email server. You will need to have a domain name pointed at your computer, plus an email server to receive mail for that domain from other servers on the Internet: