A Look at Some Sendmail Replacements

Although Sendmail is still widely recognized as the industry-leading MTA by a wide margin, it's not without its faultsin fact, it's generally understood to be rife with them. Sendmail is widely criticized for its barely comprehensible configuration files, the "master config" file in its otherwise almost-unknown m4 format, the proliferation of different runtime files (such as the access database and virtusertable file), and its comparative bulk and sluggishness. If you're comfortable with the way Sendmail works in its default configuration (which usually doesn't take much extra effort), you may have no need to complain. However, if you find you need more speed and less resource consumption, you can choose a replacement MTA. Replacements for Sendmail abound, each with its advantages, disadvantages, and die-hard supporters.

Note

A file called /etc/mail/mailer.conf controls an abstraction layer that allows you to drop in a replacement for Sendmail and have the system still operate as it did beforehand. Its content is a set of aliases or pointers that define commands such as sendmail and mailq (which are really just aliases to specific Sendmail options; for instance, sendmail bp for mailq) to refer to a specific binary, which by default is /usr/libexec/sendmail/sendmailthe actual Sendmail Consortium MTA program.

/usr/sbin/sendmail is really a symbolic link to /usr/sbin/mailwrapper, which uses the mailer.conf file to tell it how to redirect calls for those Sendmail-related commands. If you install a replacement program such as Postfix, its installation script will modify mailer.conf for you so that you can continue to type sendmail or mailq, and mailwrapper will redirect those commands off to the new Postfix binary instead of the default Sendmail. This technique also makes it easy to switch back to Sendmail if you change your mind later.

Postfix

Perhaps the largest and most compelling Sendmail alternative right now is Postfix, developed by Wietse Venema with the primary goals of speed and security. Also, its structure tends to be very "Sendmail-like," which makes it a good choice for an administrator of a large and entrenched Sendmail system that needs to be given a performance boost. However, its biggest advantage over Sendmail is that its configuration is vastly, vastly easier.

The speed of Postfix is reported to be extremely good, easily faster than Qmail, for example, although this is difficult to measure empirically. Postfix has garnered many adherents in recent years, and its feature set is now all but equivalent to Sendmail's; switching to Postfix will gain you significantly in simplicity of configuration while obliging you to endure very few sacrifices in functionality.

The Postfix home page is at http://www.postfix.org.

Qmail

Dan Bernstein wrote Qmail specifically to provide an alternative to Sendmail that was fast and secure, much the same goals as underpinned Postfix's development. Unlike Sendmail, however, the internal structure of Qmail is built up from scratch, not adapted from the Sendmail structure; this means that the configuration and the file structure for a Qmail installation don't much resemble their counterparts in Sendmail. Qmail consists of a suite of separate processes that each handle a specific part of the MTA systemdelivery, logging, queueing, and so oneach with its own permissions and execution schedule. This makes Qmail far more flexible and quick to respond, as well as easier to reconfigure on the fly, than its competitors.

Qmail is a good choice for an administrator who wants speed and security but doesn't know enough about Sendmail to be attached to it. It's also integrated tightly with the EZMLM mailing list manager, also by Bernstein, so Qmail would serve you well if you use EZMLM.

The Qmail home page is at http://www.qmail.org.

Exim

Developed at Cambridge University, Exim offers a Sendmail alternative whose chief advantage is a helpful complement of documentation, developer support, and mailing lists. It's also reputed to be very easy to configure, largely due to these sources of help, and more mature than some of the other alternative MTAs.

However, Exim has had historical problems with security, and it's one of the poorer choices of Sendmail alternatives when it comes to modularity or advanced features. Most Exim installations have been replaced with Postfix or Qmail in recent years. It's still a high-performance server, though, and can speed up SMTP services by a significant amount over Sendmail.

The Exim home page is at http://www.exim.org.