If you have a 2 node Exchange 2007 CCR cluster, you may come across a scenario where you create or remove a user account and you find that the OAB is not updating.
The most probable and common cause of this is an incorrect setting in the registry. If you open regedit on each cluster node and navigate to the following registry key:
Make sure that this parameter is set to the <active node name>. If it isn’t, just right click this registry key and modify. If this registry key does not exist then it may need to be created.