As of today, the Postfix mail transport agent has almost 50,000 lines of code, comments not included.

The Postfix security model is based on keeping software simple and stupid.

Qmail out of the box works fine, so people will want to use it regardless of licensing restrictions, even when the software does not ship with their system software.

Writing software that's safe even in the presence of bugs makes the challenge even more interesting.

At the time the Sendmail program had a very poor reputation with respect to security, with four root vulnerabilities per year for two successive years.

This will surprise some of your readers, but my primary interest is not with computer security. I am primarily interested in writing software that works as intended.

However, writing software without defects is not sufficient. In my experience, it is at least as difficult to write software that is safe - that is, software that behaves reasonably under adverse conditions.