Exchange knows several object and session limits which lead to issues on client side. You detect them usually with server side error MSExchangeIS Event ID 9646.

Possible issues:

  • Not all outlook folders are updated
  • Some folders are not visible (but visible in OWA)
  • Issues with categories
  • …

Mostly users hit the limits when they have a big folder structure (more than 500 folders) or they have attached multiple shared maiboxes.

Outlook possibly shows the status This folder was last updated on… (German: Dieser Ordner wurde um …. zuletzt aktualisiert) or This folder has not been updated (German: Dieser Ordner wurde noch nicht aktualisiert) instead of All folders are up to date (German: Alle Ordner sind aktuell).

On the mailbox server you see events like these:

MSExchangeIS Event ID 9646: mapi session “/o=First Organization/ou=Administrative Group/cn=Recipients/cn=user” exceeded the maximum of 500 objects of type “objtFolderView”
MSExchangeIS Event ID 9646: mapi session “/o=First Organization/ou=Administrative Group/cn=Recipients/cn=user” exceeded the maximum of 500 objects of type “objtFolder”.

MSExchangeIS Event ID 9646: Mapi session “sessionid” exceeded the maximum of 32 objects of type “session”.

While some limits are session limits other limits are open item limits.

Resolution

You can resolve this issue with changes in the Registry (must be done on every exchange server in the organization):

To adjust session limits:

  1. Open the Registry Editor (regedit.exe)
  2. Navigate to the path HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
  3. In ParametersSystem create a new DWORD (32-bit) Value
  4. Rename the key to Maximum Allowed Sessions Per User
  5. Set the new limit to 100 (32 is default, you can set it to 50 or 100 for example but also to a much higher value)
  6. To increase the session limit on a Client Access server the RCAMaxConcurrency value must be increased on the throttling policy too
  7. Open Exchange management shell and create a Organization wide Trottling Policy (Exchange 2013 and above):
    New-ThrottlingPolicy -Name AllUsersPolicy -ThrottlingPolicyScope Organization -RCAMaxConcurrency 4000 -EwsMaxConcurrency 80 -OutlookServiceMaxConcurrency 80 -OutlookServiceMaxSocketConnectionsPerDevice 24
  8. If a user has a dedicated policy assigned, the settings of that policy needs to be adjusted.
  9. Verify that the throttling policy you modified is assigned to the affected user mailbox or all user mailboxes depending on your needs.

 

To adjust open item limits:

  1. Open the Registry Editor (regedit.exe)
  2. Navigate to the path HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
  3. Create a key called MaxObjsPerMapiSession
  4. Create a new DWORD (32-bit) Value inside MaxObjsPerMapiSession
  5. Rename the key to objtFolder and set the value data to 1000 (500 is default, much higher values are possible)
  6. Repeat step 4 and 5 for every object type you want to increase the limit (objtFolderView, objtMessage, objtMessageView, objtAttachment, objtAttachmentView)

For the changes to take effect please restart the Microsoft Information Store.

Microsoft Knowledge Base:
https://technet.microsoft.com/en-us/library/mt741981(v=exchg.150).aspx