Why Not Use qmail?

< Free Open Study >

qmail has many advantages over other MTAs, but like any solution to a complex problem, it's not optimized for all possible scenarios. qmail was designed for well-connected hosts: those with high-speed, always-on network connectivity. Although it can be adapted through the use of the serialmail package to perform quite well on systems with slow or dial-on-demand connections, other MTAs that trade performance for bandwidth efficiency, such as Postfix, might be better suited for such installations.

Comparing qmail to Other Mailers

Table 1-2 compares qmail to some of the most common Unix MTAs.

Table 1-2: Common Unix MTAs
MTA	MATURITY	SECURITY	FEATURES	PERFORMANCE	SENDMAIL-LIKE	MODULARITY
qmail	Medium	High	High	High	Add-ons	Yes
Sendmail	High	Low	High	Low	—	No
Postfix	Medium	High	Medium	High	Yes	Yes
Exim	Medium	Low	High	Medium	Yes	No
Courier	Low	Medium	High	Medium	Optional	Yes

Sendmail-like means that the MTA behaves like Sendmail in some ways that would make a switch from Sendmail to the alternative MTA more user-transparent, such as the use of .forward files, /etc/aliases, and delivery to /var/spool/mail.

Cameron Laird's Web page compares these and other free and commercial MTAs (http://starbase.neosoft.com/~claird/comp.mail.misc/MTA_comparison.html).

Sendmail

For many years, Sendmail (http://www.sendmail.org/) was simply the Unix MTA. Sure, there were alternatives such as Smail, ZMailer, and MMDF, but Sendmail was by far the most widely used. The others offered limited advantages—Smail was lightweight, ZMailer was modular and had high performance—but every Unix distribution included Sendmail. It was powerful, mature, and the de facto standard.

By the early to middle 1990s, though, it was showing its age. There was a long line of well-publicized and frequently exploited security holes, many of which resulted in remote attackers obtaining root access to the system. The booming popularity of the Internet was driving up the rate of mail deliveries beyond Sendmail's capabilities. And although Sendmail is configurable, its configuration file syntax is legendary. One standard joke is that sendmail.cf entries are indistinguishable to the casual observer from modem line noise—strings of random characters.

Sendmail has now gone commercial—in addition to the free distribution—and continues to be actively maintained and developed. Sendmail fans like to point to its recent security track record as evidence of its security, but Sendmail's do-everything-as-root-in-one-program design is inherently insecure. All the holes in the dike might be plugged at the moment, but it might be considered imprudent to believe that others won't spring up in the future.

Nothing short of a redesign will bring Sendmail up to modern standards of security, reliability, and efficiency.

Postfix

Wietse Venema, author and coauthor of several free security-related software packages including TCP Wrappers, SATAN, and logdaemon wrote Postfix (http://www.postfix.org/) because he wasn't happy with any of the available Unix MTAs—including qmail. Postfix is a modern, high-performance MTA that shares many of the design elements of qmail while also retaining maximum compatibility with Sendmail's user interface.

Compared to qmail, Postfix is larger, more complicated, less secure, less reliable, and almost as fast. While Postfix and qmail are both modular, all of Postfix's modules run under the same user, so compromising one module could compromise the entire system. The goal of compatibility with Sendmail's user interface has limited the extent to which Venema could innovate and has saddled Postfix with Sendmail baggage like the ill-defined and hard-to-parse .forward file syntax.

Overall, Postfix is a good, solid MTA that can substitute well for qmail in most applications. If you don't demand the highest levels of security and performance, you might want to experiment with both and use the one most comfortable to you.

Courier

Sam Varshavchik, author of the Courier-IMAP daemon often used with qmail, wrote Courier (http://courier.sourceforge.net/) because he wasn't happy with any of the available Unix MTAs—including qmail and Postfix.

Courier is an integrated suite of mail servers that provide SMTP/ESMTP, IMAP, POP3, Web mail, and mailing-list services. Most MTAs only provide SMTP/ESMTP service. qmail includes a POP3 server. Courier's IMAP server is often used with qmail because it supports qmail's maildir mailbox format.

Courier is still in beta release. The author considers it reliable and essentially complete, but not fully mature.

Exim

Philip Hazel developed Exim (http://www.exim.org/) at the University of Cambridge. It was intended to be small and simple, like Smail, but with more features. It has many modern features, but like Sendmail, is monolithic. Security and performance were not primary design goals. In many respects, Exim is comparable to Sendmail but is not nearly as widely used.

< Free Open Study >