If you use in your transport rules filter conditions based on AD group membership, you should be aware that the MS Exchange transport service caches group membership information by default for 4 hours. This means that if you change group membership, it may take up to 4 hours before the changes are effective. To force an immediate update of the group membership cache, you must stop and start the Microsoft Exchange Transport service. You must do this for each Hub Transport server where you want to forcibly update the recipient cache.

MXS 2007: The cache interval settings cannot be modified.

MXS 2010: The cache interval settings can be modified, but the Exchange documentation contains no information about how to do this. You may use an article in the documentation for Operations Manager Management Pack for Exchange 2010 for information about available settings and how to configure them: Transport IsMemberOfResolver ExpandedGroups Cache nearing capacity - Yellow(>66) 

-Franz Krainer