From Computer Tyme Support Wiki
Qmail is an old and obsolete MTA (Mail Transfer Agent) that was developed in the mid 1990s to be a better email program than Sendmail. In its day it did the job but Qmail became fossilized in June of 1998 when its author Dan J. Bernstein dropped the ball and stopped developing it.
Although Qmail is a free program it is free as in beer, and in the past, it has not been free as in freedom.
In November of 2007, it was released into the public domain; however, originally, Qmail was released under a bizzare and improper license that restricted the way patches are distributed and forced users to work with only DJB's version of the program. His license restricted forking the code and letting the community take over developing it and since he had abandoned it, the product is dead save for a small cult of fans and old systems developed around it who haven't yet figured out how to get rid of it.
One of the major problems with Qmail is that it isn't RFC compatible in the way it retries MX records on failure to deliver. If I have 3 servers and my main server is returning a temporary error, every other MTA will then send email to the backup servers as the specifications dictate. Every MTA except for Qmail. Qmail will retry forever on the failed MX record until it times out and gives up. This make Qmail incompatible with some really good spam filtering techniques that only work with MTAs that play by the rules. Qmail doesn't play by the rules.