For many people my software is something that you install and forget. I like to keep it that way.
As of today, the Postfix mail transport agent has almost 50,000 lines of code, comments not included.
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.
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.
I don't expect an overnight change of all desktops to what the US Military used to call B3 level security. And even that would not stop users from shooting themselves into the foot.
Postfix keeps running even if one Postfix process dies; Windows requires that someone restarts the service.
The challenge with Postfix, or with any piece of software, is to update software without introducing problems.
Defect-free software does not exist.
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.
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.