Exchange EventID 9646 , Mapi Session Exceeded

As users mailboxes grow or they add additional accounts to the same outlook instance such as shared mailboxes or old employees. The number of folders that the user has open from that session can exclude the number of folders allowed by exchange per session. This can cause very weird behavior, missing emails, lost calendar appointments as outlook is not able to sync every folder that is open. Here is an example of the error from the MSExchangeIS service:

Mapi session “{GUID}: /o=First Organization/ou=Exchange Administrative Group (ORGID)/cn=Recipients/cn=James Bond” exceeded the maximum of 500 objects of type “objtFolder”.

Fixing the error is easy, increase the maximum number of objects that MAPI allows:

  1. Open the Regedit and navigate to
    1. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\MSExchangeIS
  2. Create a new key(Folder)
    1. MaxObjsPerMapiSession
  3. Create two new dwords
    1. objtFolder
    2. objtFolderView
  4. Populate the values with number that will suit your needs, 750. Make sure to switch the DWORDS base to decimal otherwise the values will be very high.

Optional – If you need to force the values. Restart the information store service that will reset user connections in a non-DAG environment. Otherwise the store will pickup the changes in a period of time, sometimes the MAPI connection(outlook) will need to be closed and reopened as well.

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