I got beat with this in my new Exchange 2010 / BES 5.0.2
Could not activate users, until I called Microsoft, and they gave me: KB 842022, and I increased the MAPI session:
DWORD: Maximum Allowed Sessions Per User = DEC.128
Event ID 9646 is logged in the application event log of your Exchange Server 2003 computer when a client opens many MAPI sessions