SMTP Server Options for Linux

Quite a few SMTP servers are available for Linux. The four most popular are:

• Sendmail ” The mail server with the largest installed base, and the one that ships with most Linux distributions, is sendmail. This package is large and powerful, and many programs assume that sendmail is available, so other packages usually include a binary called sendmail to maintain compatibility. Sendmail uses a complex configuration file format, which is one of the reasons alternatives have been growing in popularity. The main sendmail Web site is http://www.sendmail.org.

• Exim ” Exim is a mail server that was designed to use a simpler configuration file format than sendmail, and to support various sophisticated filtering operations on mail. It's the favored mail server with Debian and its derivative distributions. The main Exim Web site is http://www.exim.org.

• Postfix ” Both sendmail and Exim are monolithic mail servers, meaning that they perform most tasks in a single large program. Postfix was designed in a modular way, meaning that tasks are broken up and handled by smaller programs. This has certain security and speed benefits. These and Postfix's simpler configuration files are its main advantages over sendmail. Linux Mandrake ships with Postfix as the default mail server. You can learn more about Postfix at http://www.postfix.org.

• qmail ” The qmail server, like Postfix, is a modular server designed with security and performance in mind. The qmail configuration file is simpler than that of sendmail, but the program is less compatible with sendmail than either Exim or Postfix, so it's a bit more difficult to replace sendmail with qmail. Although it's turned up as the second most popular UNIX and Linux server in surveys of Internet mail servers, qmail isn't the default mail server for any Linux distribution, so I don't discuss it at length in this chapter. The main qmail Web site is http://www.qmail.org.

Although these are the most popular Linux mail servers, there are others, such as Smail (http://www.gnu.org/software/smail/smail.html), Courier (http://www.courier-mta.org), and OpenMail (http://www.openmail.com/cyc/om/00/). Most of these are open source, but some are commercial. The "big four" account for most of the discussion of mail servers in the Linux community. All four of these programs are very powerful and able to handle the mail needs for most domains ”even very large ones.

Particularly if you're new to mail server administration, you're probably best off using whatever SMTP server shipped with your distribution. Many distributions now include multiple SMTP server packages. The default is probably the best choice.

If you have specific needs, and particularly if you have exotic needs, you may want to investigate the capacity of various mail servers to meet those needs. If necessary, you can replace your standard mail server with another one. Because of sendmail's popularity, this usually means replacing sendmail with another server. Exim and Postfix usually work well as "drop-in" sendmail replacements . Although the configuration files are completely different, programs that call sendmail directly usually work well with Exim and Postfix, and the mail queue format for these two programs defaults to the same format that sendmail uses ”namely, the mbox format, in which all mail in a single mail "folder" is stored in a single file. Replacing sendmail with qmail is usually a bit more involved, because qmail defaults to a different mail file format (the maildir format, which uses a directory in which messages are stored as individual files), so you may need to change the standard qmail configuration or replace your mail programs (including any pull mail servers you want to run, as discussed in Chapter 11).