The Microsoft Forefront Security for Exchange Server (FSE) keyword filter stops working if a keyword filter list contains over 32,767 entries. No keyword detections will be made when the number of keyword filter list entries exceeds 32,767 entries.
To resolve this problem, install Forefront Security for Exchange Server with Service Pack 2 (SP2).
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
Description of Forefront Security for Exchange Server with Service Pack 2
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.