ClamSMTP is an SMTP filter that allows you to check for viruses using
the ClamAV anti-virus software. It accepts SMTP connections and forwards
the SMTP commands and responses to another SMTP server. The 'DATA' email
body is intercepted and scanned before forwarding.

ClamSMTP aims to be lightweight, reliable, and simple rather than have a
myriad of options. It's written in C without major dependencies. If you
need more options then you could use something big like AMaViS which is
written in PERL and can do almost anything.