Many of the plugins mentioned in Matt's article, including virus scanning and spam filtering, are enabled by default and qpsmtpd makes it easy to enable others.

The SME Server development team has also contributed a few plugins to the qpsmtpd project.

http://sourceforge.net/projects/smeserver/

http://www.planetmirror.com/pub/smeserver/releases/7/iso/

http://www.contribs.org/

Gordon