Qsmtp is a drop-in replacement for qmail-smtpd and qmail-remote.
The benefits:

- Fast and small: written in plain C
- IPv6 clean
- Many antispam modules included, e.g. SPF, IP blacklists, domain
  blacklists, badmailfrom, helo filtering
- Easy API to add your own spamfilters
- For vpopmail hosts: does not create bounces for non-existing users
- Tested: includes unit tests, nightly checks available at
  <http://my.cdash.org/index.php?project=Qsmtp>

Homepage:
https://opensource.sf-tec.de/Qsmtp/
