After installing Exchange 2010 SP3 OWA gives the error "A problem occurred while you were trying to use your mailbox."

An odd problem post Exchange 2010 SP3, I tested OWA and all was fine, then a user called to say he was getting an error after a random number of clicks and sometimes immediately after login. It turns out there is a limit on the number of non-MAPI connections and this was locking out OWA.

The Microsoft article does not give a value for the registry key, I set mine to 100 Decimal, so far so good.


You cannot connect to an Exchange Server 2010 mailbox from Outlook Web Access:

1. On the server that is running the Exchange Server 2010 Mailbox role, click Start, click Run, type regedit, and then clickOK.
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type Maximum Allowed Service Sessions Per User, and then press ENTER.
5. On the Edit menu, click Modify.
6. Type the decimal value that specifies the number of sessions that you want to use, and then click OK.
7. Exit Registry Editor.

Note The registry value Maximum Allowed Service Sessions Per User affects all Mailbox Databases on the server and is not applied on a per-user basis. If you increase this value, server performance may be adversely affected. For example, doubling the number of service sessions from 16 to 32 could slow server performance.