Event ID: 9646 Mapi session exceeded the maximum if 500 objects of type “objtFolder”

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Outlook is not updating folders automatically on machine with more than one active mailbox.

Server shows Event ID: 9646 in logs.

Regedit
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\MSExchangeIS\ParametersSystem

New Key: MaxObjsPerMapiSession
New DWORD: objtFolder value of 1000
New DWORD: objtFolderView value of 1000

You do not need to restart exchange.

Source: The Intengrity