Red Hat ENTERPRISE LINUX 3 Reference Manual

Also see for ENTERPRISE LINUX 3 - SECURITY GUIDE: Tuning guide Installation guide Installation guide Installation guide Manual

Contents

162 Chapter 11. Email11.2.3. Mail User AgentA Mail User Agent (MUA) is synonymous with an email client application. An MUA is a programthat, at the very least, allows a user to read and compose email messages. Many MUAs are capableof retrieving messages via the POP or IMAP protocols, setting up mailboxes to store messages, andsending outbound messages to an MTA.MUAs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface, such asmutt.11.3. Mail Transport AgentsRed Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is configuredas the default MTA, although it is easy to switch the default MTA to Postfix.TipFor information about how to switch the default MTA from Sendmail to Postfix, refer to the chaptercalled Mail Transport Agent (MTA) Configuration in the Red Hat Enterprise Linux System Administra-tion Guide.11.3.1. SendmailSendmail’s core purpose, like other MTAs, is to safely transfer email among hosts, usually usingthe SMTP protocol. However, Sendmail is highly configurable, allowing control over almost everyaspect of how email is handled, including the protocol used. Many system administrators elect to useSendmail as their MTA due to its power and scalability.11.3.1.1. Purpose and LimitationsIt is important to be aware of what Sendmail is and what it can do as opposed to what it is not. In thesedays of monolithic applications that fulfill multiple roles, Sendmail may seem like the only applicationneeded to run an email server within an organization. Technically, this is true, as Sendmail can spoolmail to each users’ directory and deliver outbound mail for users. However, most users actually requiremuch more than simple email delivery. They usually want to interact with their email using an MUA,that uses POP or IMAP, to download their messages to their local machine. Or, they may prefer aWeb interface to gain access to their mailbox. These other applications can work in conjunction withSendmail, but they actually exist for different reasons and can operate separately from one another.It is beyond the scope of this section to go into all that Sendmail should or could be configured todo. With literally hundreds of different options and rule sets, entire volumes have been dedicated tohelping explain everything that can be done and how to fix things that go wrong. Refer to the Section11.6 Additional Resources for a list of Sendmail resources.This section reviews the files installed with Sendmail by default and reviews basic configurationchanges, including how to stop unwanted email (spam) and how to extend Sendmail with theLightweight Directory Access Protocol (LDAP).11.3.1.2. The Default Sendmail InstallationThe Sendmail executable is /usr/sbin/sendmail.