The Design of QMail

Here are some excerpts about QMail from the Wiki and from the QMail web site.

qmail is the second most popular mail transport agent, behind sendmail. It was written in reaction to sendmail. Its main design goal was to be secure, and in its seven years of existence, has never had a security flaw. One of the keys to meeting this goal is by being made up of about twenty-five simple C programs. The biggest one is 1600 lines long, the others are 200-600. Bernstein is a genius programmer and qmail is a masterpiece.

I find it interesting that the system is using the lots of small programs pattern. It's used here for security. Berstein explains that he runs as little code as possible in setuid mode. So even if a particular program is hacked, the compromise is not be deadly. The security is not all-or-nothing.