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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 sendmailsystem, first distributed for free with the Unixoperating 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: