Certification Objective 12.01-Mail Transport Agents, Mail Delivery Agents, and Mail User Agents

When you install sendmail, Postfix, and/or Dovecot, you also get huge and difficult-to-read configuration files. Do not be intimidated, as it's likely that you'll have to change only a few entries in each file.

Definitions

A mail server has three major components, as described in Table 12-1. You need all three components to have a fully functional mail system. Fortunately, as the other components are already installed, you should have to install only the MTAs that you need on a standard RHEL system.

Table 12-1: Mail Server Components

Abbreviation	Meaning	Examples
MTA	Mail transfer agent	sendmail, Postfix, Dovecot
MUA	Mail user agent	mail, Evolution, elm
MDA	Mail delivery agent	procmail, maildrop

On any Linux computer, you can configure some mail transfer agents (sendmail or Postfix) for various outbound services, such as forwarding, relaying, method of transport (such as TCP or UDP), lists of computers with other MTAs, optional aliases, and spooling directories. Others, such as Dovecot, are designed to handle only incoming e-mail services.

E-mail systems are heavily dependent on name resolution. While you could handle name resolution through /etc/hosts on a small network, any mail system that requires Internet access needs access to a fully functional DNS server.

The sendmail and Postfix systems use SMTP to send e-mail. But that is only one end of the mail system. You also need to use a service such as Dovecot to enable POP3 and/or IMAP (or the secure cousins, POP3s and IMAPS) to receive e-mail.

On the Job 

While this chapter refers to the IMAP and IMAPS protocols, rest assured that these options support the current versions of these protocols, IMAP4 and IMAP4S.

SMTP, the Simple Mail Transfer Protocol, has become one of the most important service protocols of the modern era. Much of the Internet-connected world lives and dies by e-mail and relies on SMTP to deliver it. Like POP3 and IMAP, SMTP is a protocol, a set of rules for transferring data used by various mail transfer agents.

Installing Mail Server Packages

The RPM packages associated with sendmail and Postfix are both part of the Mail Server package group. Key packages are listed in Table 12-2. You can install them with the rpm or yum command. Just remember that you may not need to install everything in this table.

Table 12-2: Key Mail Server RPMs

RPM Package	Description
cyrus-imapd*	Installs the Cyrus IMAP enterprise e-mail system (several packages); may require perl-Cyrus
cyrus-sasl	Adds the Cyrus implementation of the Simple Authentication and Security Layer (SASL)
dovecot	Supports both the IMAP and the POP incoming e-mail protocols
exim	Adds another MTA; another alternative to sendmail and Postfix
mailman	Supports e-mail discussion lists
postfix	Includes an alternative to sendmail
sendmail	Installs the most popular mail server of the same name
sendmail-cf	Adds a number of templates that you can use to generate your sendmail configuration file
spamassassin	Includes the spam fighting package of the same name
squirrelmail	Installs a Web-based e-mail server
system-switch-mail system-switch-mail-gnome	Adds a GUI method for switching between sendmail and Postfix

When you install the default Mail Server package group, you're installing the sendmail and Dovecot packages. Since you may not need all of these packages, it may be faster to install these with the rpm or yum command, especially if you're configuring your Linux computer from the text console. It takes time to start the GUI.

On the Job 

You can find a list of RPMs associated with each package group on the first installation CD in the /Server/repodata directory in the comps-rhel5-server-core.xml file. If you're running the RHEL 5 desktop, substitute "Client" for "Server."