In a previous life I wrote the software that controlled my physics experiments. That software had to deal with all kinds of possible failures in equipment. That is probably where I learned to rely on multiple safety nets inside and around my systems.

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

I want to avoid locking people into solutions that work only with Postfix. People should have a choice in what software they want to use with Postfix, be it anti-virus or otherwise.

One bug in an SMTP server can open up the whole machine for intrusion.

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.

The challenge with Postfix, or with any piece of software, is to update software without introducing problems.

Adding functionality is not just a matter of adding code.