Qmail-scanner
From Qmail Info Wiki
qmail-scanner is a program which sits between qmail-smtpd and qmail-inject, in order to perform filtering functions. It can be configured to call one or more virus scanners, as well as spamassassin. It also has a very basic set of filters built into itself.
Because it is written in Perl, it is rather easy to customize. (add info on that spamassassin bypass patch here)
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Links
Sourceforge page for qmail-scanner (http://qmail-scanner.sourceforge.net/) (which is also the official home page for qmail-scanner)
Freshmeat page for qmail-scanner (http://freshmeat.net/projects/qmail-scanner/)
