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.
Wietse VenemaPostfix keeps running even if one Postfix process dies; Windows requires that someone restarts the service.
Wietse VenemaMost of the effort in the software business goes into the maintenance of code that already exists.
Wietse Venema