ClamAV ineffective - not blocking virii???

I've been received virus emails from a few people over the past few months and have clamav running on the server and scanning all incoming mail and it's never detected some of these viruses like the Win32/Mytob.DJ worm that my workstation AV (nod32) always catches..

My concern is that ClamAV is not blocking (m)any virueses with the default cpanel config/installation ..or is it?

Does anyone know how to improve clamav's performance? Keep in mind I'm using the clamavconnector module in WHM that installs Clamav (0.87 installed)..