[uxbod]

You may wish to beef up your AS protection aswell; so here are a couple of things to read through :-

Improving Anti-spam system - Zimbra :: Wiki
[SOLVED] SaneSecurity ClamAV or FuzzyOCR SpamAssassin Plugins
BarracudaCentral.org - Technical Insight for Security Pros
Sanesecurity's ClamAV - Phishing and Scam/Spam Signatures