Simscan is a VERY fast, small, lightweight email scanner written in pure C. Works with Trophie (if present), clamav (if present), spamassassin (if present). It is the current scanning agent of choice.
patch for custom SMTP reject messages
You will however need to use a different version of the qmail-queue-custom-error.patch from the one provided in /contrib at inter7.com if you're running a FreeBSD box and have selected all the options I normally select when building Qmail from ports - something about "qqargs" breaks the bone-stock patch.
Here's the fixed patch: File:Qmail-queue-custom-error.patch
common oopses: 451 with no /var/log/message entries
DO NOT forget to make clamav run as root! Despite checking the box in the port, as of Nov 2007 the default clamd config you end up with still runs as clamav, which CANNOT scan the necessary directories and will produce no mail and no entries in the maillog. And much frustration.
DO NOT forget to run /var/qmail/bin/simscanmk -g after installing! If you have not done so, and you have enabled adding headers to show the message scans, SIMSCAN WILL FAIL. Sigh.