Ignoring the elephant in the room.....

The Qmail restriction was that it had to be redistributed exactly as you got it, whether pristine source or a binary you built from that pristine source. Much of the commonly used functionality cam from patches, leading to ugly "netqmail" tarballs which included a subset of a confusing and conflicting master set of patches and the code to apply them. You were not allowed to distribute a binary built from patched source.

This restriction has since been removed. Qmail was relicensed (?) as public domain software a year or so ago.

I probably have some minor details wrong, but qmail was never free source software in any traditional meaning of the term.