http://mmmservices.web.cern.ch/mmmservices/Help/contents/0_4_spam-filters.asp
A nice explanation of the various filters being applied by one hosting service.
1. Checking the validity of the envelope sender address
2. Well known spam destinations
3. Attempts to use CERN as a relay are rejected
4. Well known dangerous attachments
5. Bad local addresses
6. BadStrings (viagra)
