The e-mail server software used to transmit e-mail across the Internet is called a Mail Transfer Agent, or MTA. There are several MTAs that run well on Linux: Sendmail, qmail, PostFix, and Exim among others. Sendmail is by far the most popular MTA on the planet and is the default MTA under Fedora Core. In this chapter, therefore, we focus on Sendmail configuration and troubleshooting. However, some users may prefer the PostFix MTA (also included with Fedora Core). At the end of this chapter, we introduce PostFix and provide basic information on configuring and running this alternative server.
Sendmail is released in two versions: commercial and Free (speech, not beer). In this chapter, we address the Open Source version shipped with Fedora Core. If you need enterprise-level e-mail management, consider the sendmail.com commercial version. It provides a graphical interface, integrated spam control, and transparent antivirus protection.
You should know that Fedora Core or other Red Hat Linux installations do not automatically configure MTAs to detect incoming mail from the Internet. That is, the SMTP service or daemon binds only to localhost by default, so the mail server does not see incoming mail from the network. In order to get and distribute mail on your network, you'll need to configure your MTA of choice. Note that, by default, MTAs are designed only to gather mail from local scripts, programs, or users, and then to transfer those messages to other e-mail accounts (local or remote). If you want to run an MTA that listens to traffic from the network or the Internet, you must enable those functions. This is a security matter.