Qmail has either absolute devotees, or those who deride it as being old
fashioned and out of sync with today's world. It was written by
Daniel J Bernstein (DJB to most people) in the mid 1990's and has
come through 15 years virtually unscathed. Currently it is (depending
on which survey) the sixth most commonly used smtp server, following
not far behind Microsoft Exchange in percentage terms. This is a
fairly remarkable achievement for a piece of software that is not
marketed or promoted by anyone.
Most of those who dislike it, dislike it for its lack of built in features.
This lack is its very strength, as it has survived for this period of
time without needing security patches, an unusual phenomena in this
day and age.
There are clear indications that Yahoo one of the three biggest E-mail
service providers in the world still use Qmail, and they provide the
smtp services for many other service providers for example in the UK,
BT's smtp servers show signs of being run by Yahoo. I am therefor
sure that if we had to account by the volume of Email being handled,
Qmail is probably in the top 3 smtp servers world wide.
A lot of the detractors have held their positions because because of
DJB's insistance that while the software was free to anyone to use no
one had a right to make changes. You could add anything you liked but
were not allowed to tamper with the base code. It has been this
restrictive policy that has kept Qmail from needing lots of security
fixes. The program is now in the public domain and although DJB no
longer insists on no tampering >he advises against it.