Quoting Steve Hanson (shanson at cruiskeen.com):
> I've used mailscanner in the past - it does require that you use one of 
> the commercial virus scanner products that run on linux - mcafee or 
> Sophos or some such
> 
>   http://www.sng.ecs.soton.ac.uk/mailscanner/

Nate has been testing MIMEDefang, I'll let him comment.
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