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 Zweitze Venema (born 1951) is a Dutch programmer and physicist best known for writing the Postfix email system. He also wrote TCP Wrapper and collaborated with Dan Farmer and Samuel Johnson to produce the computer security tools SATAN and The Coroner's Toolkit.