MSExchangeIS EventID 9646: Mapi session exceeded the maximum of 500 objects of type “objtFolderView"

On Exchange you might see the following error logged, event ID 9646.  This error is caused by a users mailbox having over 500 folders and sub-folders. Here is the event log:

Solution:

To resolve this issue create the following registry keys on each of your mailbox servers:

• Navigate to HKLM > System > CurrentControlSet > Services > MSExchangeIS > ParametersSystem.
• Create a new key in ParametersSystem named MaxObjsPerMapiSession.
• In MaxObjsPerMapiSession, create a DWORD named objtFolder with a decimal value of 600.
• Create a second DWORD named objtFolderView also with a decimal value of 600.

The Information Store will pick up the changes after a few minutes. No need to reboot the Informational Store.

Hope it helps.