Users that have access to a number of shared mailboxes in their Outlook may notice the contents of those shared mailboxes are no longer synchronized properly. Their own personal mailbox does not seem to have this problem. On the server running the Exchange Information Store (MSExchangeIS), a variation of the following event is logged when this occurs:

Event ID 9646

Mapi session "ba765653-5439-437a-993f-806575b85fbb: /o=My Company/ou=First Administrative Group/cn=Recipients/cn=user" exceeded the maximum of 500 objects of type "objtFolder".

The reason for this error is that the Outlook client has hit the maximum number of MAPI connections allowed for the specified object type (in our example: objtFolder). Subsequent connections are blocked, causing Outlook to no longer synchronize properly. The Outlook client is more likely to hit this limit as there are more shared mailboxes that have a lot of folders in them, because the synchronization of those folders requires Outlook to create more connections to the server, possibly hitting the maximum MAPI connection limit.

You can permanently resolve the issue by increasing the number of maximum allowed MAPI connections for the types that exhibit the problem. This is done on the server running the Exchange Information Store. If you have a DAG (Database Availability Group), you need to perform this on each of the servers running the Exchange Information Store.

1. On the server, start up regedit.
2. Navigate to the registry key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
3. If not present, create a new key called MaxObjsPerMapiSession.
4. Inside MaxObjsPerMapiSession, create new DWORD (32-bit) values for each of the MAPI object types you wish to increase:
   • Name = the name of the MAPI object type (e.g. objtFolder).
   • Value = the maximum number of connections allowed for this type, per user.
   • Type = DWORD (32-bit)

   Although there are many more, these are the most common you're likely to encounter. If a type is not specified in the registry, it will divert to its default value (2nd column).

   Name	Default
   objtMessage	250
   objtFolder	500
   objtAttachment	500
   objtFolderView	500
   objtMessageView	500
   objtAttachView	500

   You only need to create a registry value for each of the type you wish to change from the default value. Those that are not specified will keep using the default value. When increasing values, be sure to do it gradually, as increasing values will increase the load on your mailbox server(s). It is not possible to increase the limits for a single or a group of users. Changes are for all users on that specific mailbox server.
5. When finished, restart the service Microsoft Exchange Information Store.

Client-side workaround

If you are hesitant, or unable to make changes to the maximum allowed MAPI connections, you can also work around the issue on the client-side by disabling caching of shared mailboxes in Outlook.

Disabling cache for shared mailboxes alone will still allow the user to benefit from the cache for his/her personal mailbox, but not for shared mailboxes. Because Outlook no longer caches the other mailboxes, no additional MAPI connections will be established, and thus, the connection limits will not be reached.