Qpsmtpd started as a replacement daemon for the SMTP receiver (qmail-smtpd)
from the qmail mail transport agent (MTA). qmail-smtpd has a number of
shortcomings (e.g. being unable to check the validity of a recipient mail
address) and is written in C which makes it burdensome to modify and extend.
Qpsmtpd, on the other hand, is written in pure perl and can be customized
easily. It consists of a core that implements a complete SMTP server, and a
number of plugins/modules which control the operations. Such plugins
include plugins to check the recipient and sender as well as plugins for
virus scanning, spam checking, blocking lists (dns and rhs), AUTH and TLS.
Qpsmtpd can not only be used with qmail but also with e.g. postfix and
exim. It can also write messages to a Maildir or forward it to a remote
host without buffering.