Q: How do you set server side content filtering rules (Exchange 2007+)?
ANSWER: On the Hub Transport Exchange Server if you launch the Exchange Management Console (EMC) and go to – Org Config – Transport Rules. You’ll have limited abilities to filter to non-managed folders. Managed folders filtering requires additional Exchange Server CAL licensing.
http://msexchangeteam.com/archive/2006/10/25/429345.aspx