Chapter 11: Sendmail

Sendmail is the de facto standard mail transfer agent, or MTA, in use on the Internet today. While there are now several worthy contenders for the title of best or most popular MTA, including Postfix and QMail (both of which have very good Webmin modules, and Postfix is documented in the preceding chapter), more mail probably passes through Sendmail than any other single MTA.

An MTA is the software that provides mail services for a network. A client mail user agent, or MUA, sends email, usually via the Simple Mail Transport Protocol, or SMTP, to the MTA. The MTA uses one of several transport protocols, most often via SMTP, to deliver it either directly to the recipient (if the address is served by the same server) or to the mail server for the user. Clients then access the mail on the server using either POP3 or IMAP. So, Sendmail will operate on your server and provide those intermediary services, both sending and receiving mail, for clients and other MTAs on the Internet.

Configuring Sendmail

Sendmail has a reputation, not entirely undeserved, for being extremely obtuse and confusing to configure. The famously terse sendmail.cf file was designed to be easy and quick for the computer to parse, not for humans to be able to read and edit. Relatively recently, attempts have been made to remedy this problem, and the solution now provided with Sendmail is an “m4” macro–based configuration file, called sendmail.mc by default, that allows you to use much more human comprehensible configuration constructs. This configuration file is also supported by Webmin in the Sendmail M4 Configuration page.

Sendmail also uses a few other configuration files to dictate certain other behaviors. These include aliases, mailertable, access, and domaintable. These files are quite readable by mere mortal humans, usually containing a few (or more than a few in large networks) names, hosts, or domains, and an option or permission that applies to the name, host, or domain. Webmin provides a nice interface for these files as well, so you won’t have to deal with them directly. However, it is good to know about them and what they’re used for. You’ll learn about them in more detail as the relevant Webmin sections are discussed.