Personally I use amavisd, with AntiVir from H+B EDV (www.hbedv.de), trophie (http://www.vanja.com/tools/trophie/), an alternative frontend for TrendMicro's libvsapi shared library, and clamav (http://clamav.elektrapro.com/), a free scanner which uses OpenAV's (http://www.openantivirus.org) signatures, but is implemeted in C (openav is written in Java). All these in different combinations, and they all work very well. I must say that amavisd is a really outstanding piece of software, and can only recommend using it.
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If you only use the H+BEDV scanner (AntiVir), you can also use their 'avmilter' package. It is available for download on their homepage (http://www.hbedv.com).
Avmilter is fine, but does not allow certain sites. It goes between sendmail or postfix and does act as smtp. I would not use it, because the features of your original mail transport are bypassed. Another thing is use more than one mailscanner, because the antivir virus scanner does know less viruses than some other products. Philippe