Chapter 24: Mail Servers: SMTP, POP, and IMAP

Overview

Mail servers provide Internet users with electronic mail services. They have their own TCP/IP protocols such as the Simple Mail Transfer Protocol (SMTP), the Post Office Protocol (POP), and the Internet Mail Access Protocol (IMAP). Messages are sent across the Internet through mail servers that service local domains. A domain can be seen as a subnet of the larger Internet, with its own server to handle mail messages sent from or received for users on that subnet. When a user mails a message, it is first sent from their host system to the mail server. The mail server then sends the message to another mail server on the Internet, the one servicing the subnet on which the recipient user is located. The receiving mail server then sends the message to the recipient's host system.

At each stage, a different type of operation takes place using different agents (programs). A mail user agent (MUA) is a mail client program, such as mail or Elm. With an MUA, a user composes a mail message and sends it. Then a mail transfer agent (MTA) transports the messages over the Internet. MTAs are mail servers that use SMTP to send messages across the Internet from one mail server to another, transporting them among subnets. On Linux and Unix systems, the commonly used MTA is Sendmail, a mail server daemon that constantly checks for incoming messages from other mail servers and sends outgoing messages to appropriate servers. Other MTAs becoming more popular are Postfix, Exim, Courier, and Qmail (see Table 24-1). Incoming messages received by a mail server are distributed to a user with mail delivery agents (MDAs). Most Linux systems use procmail as their MDA, taking messages received by the mail server and delivering them to user accounts (see www.procmail.org for more information).

Table 24-1: Mail Transfer Agents

Agent	Description
Sendmail	Sendmail mail transfer agent www.sendmail.com
Postfix	Fast, easy to configure, and secure mail transfer agent compatible with Sendmail and designed to replace it www.postfix.org
Qmail	Fast, flexible, and secure MTA with its own implementation and competitive with Postfix www.qmail.org
Exim	MTA based on smail3 www.exim.org
Courier	Courier MTA www.courier-mta.org
Mail Transport Agent Switcher	Red Hat tool to let you switch between using Sendmail and Postfix (Mail Transport Agent Switcher in System Settings menu and window)