How to trace and fix a MAPI error 9646 in Exchange

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The error below indicates that the MaxObjsPerMapiSession object in HKLM > System > CurrentControlSet > Services > MSExchangeIS > ParametersSystem has exceeded the limit:

Mapi session “92d354d5-d7e7-403e-98f9-cbe4d35a9838” exceeded the maximum of 500 objects of type “objtFolder“.

To find out the user mailbox, download ADFIND and run the following command:

adfind -gc -b “” -binenc -f ” msExchMailboxGUID={{GUID:92d354d5-d7e7-403e-98f9-cbe4d35a9838}}” -dn

Once you know who the user mailbox belongs to, you can run a script to count and verify how many objects there are for the mailbox.

(Get-MailboxFolderStatistics -id %username%).count

As needed, you can adjust the objtFolder and objtFolderView parameters in the registry.
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem.

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