One of the changes that Microsoft made to Windows 2008 domains involved the maximum number of MAPI sessions per user that is allowed. Of course, Microsoft have been trying to deprecate the use of MAPI for some years now but many vendors are still using it.
An example would be Blackberry Enterprise Server and GOOD Mobile Messaging.
After upgrading a domain to Windows 2008, these systems may typically start to show issues as their connections get refused. The solution is to add a new registry key named 'NSPI max sessions per user' which is DWORD and sits in
and to ensure this is set to sufficient users. Remember that some software (example Blackberry Enterprise Server) may use any global catalogue server, so in a multi domain forest, don't forget do configure every GC. The following Microsoft article provides more detail: