Most of the effort in the software business goes into the maintenance of code that already exists.
My reply is: the software has no known bugs, therefore it has not been updated.
Like all software, Qmail can survive only when it keeps up with changing requirements.
The Postfix security model is based on keeping software simple and stupid.
Like most people, I am not smart enough that I can design software without having to restructure the software every now and then.
Defect-free software does not exist.