Objective 3: Mail Transfer Agent (MTA) Basics

The four main MTAs commonly available on Linux systems are sendmail, postfix, qmail, and exim. Each has its own differences, mainly with regard to the format of configuration files. Each MTA performs the basic functions of a mail transfer agent: the sending and receiving of Internet mail.


Sendmail was one of the first MTAs used on Unix systems. It was derived from the original program “delivermail,” which shipped with an early version of BSD Unix in 1979. Sendmail has grown over the years into quite a complicated program—as evidenced by the O’Reilly book sendmail, Fourth Edition, which weighs in at a whopping 1,312 pages—and is often quite challenging to configure correctly. That fact, combined with the history of security vulnerabilities that have plagued sendmail over the years, has caused its popularity to decrease over the last decade. Although most major Linux distributions provide a package for sendmail, none of them currently ship with sendmail as the default MTA.


Postfix was originally designed in the late 1990s as a more secure alternative to sendmail. It shares many of the same configuration options as sendmail, but does not share any code. At the time of this writing, postfix is currently very popular in the Linux world, and is the default MTA shipped with the most popular Linux distributions.


In response to the increasing number of security incidents involving MTAs, qmail was developed in the mid 1990s to ...

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