The release philosophy conflict is interesting. Very rarely do we see "finished" software, but the current "official release" version of qmail was released on June, 15, 1998. This is basically due to DJB's insistence on avoiding bugs by taking the UNIX philosophy as far as possible; everything is done by a small set of processes communicating over pipes, and the attack surface kept very small.
The downside is its limited integration with other systems and especially databases. If you want to configure qmail, it has to be through text files. But if you want to operate one of the common cases of a relay-only host or a single UNIX host delivering mail to its local users, it works very well.