As part of performance optimisation for Forefront Server Security for Exchange 2007, certain filtering features are disabled on different scanjobs. In terms of the Forefront Server Security Administrator client you will see these options, but they will be greyed-out and are not configurable. Here are the main differences between Edge/Hub and Mailbox servers:
Edge and Hub Servers: This is where we find the Transport Scanjob. Keyword Filtering is available here, but you’ll find Content Filtering greyed-out.
Mailbox Servers: The Realtime Scanjob gives you the option to filter-out Content (comprising of Subject Line and Sender/Domain Filtering), but Keyword Filtering is not available (greyed-out).
You can also find more information about Content Filtering and Keyword Filtering in the Forefront Server Security User Guide.