Error: Outlook Was Unable to Recover Some or All of the Items in this Folder

While attempting to restore items from the Outlook recoverable deleted items you may encounter the following error message.

Outlook was unable to recover some or all of the items in this folder. Make sure you have the required permissions to recover items in this folder, and try again. If the problem persists, contact your administrator.

The error may occur when you attempt to recover a folder that contains a large number of sub-folders or mail items in it.

On the Exchange Server you may see a corresponding Application event log entry similar to this:

Log Name:      Application
Source:        MSExchangeIS
Date:          11/14/2011 12:57:59 PM
Event ID:      9646
Task Category: General
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      exchangeserver.domain.local
Description:
Mapi session "3bbea118-07e3-4746-b7bf-0e5b65e28314" exceeded the maximum of 500 objects of type "objtFolder".

This can occur due to the default limits configured on the Exchange Server for the number of items that can be opened by a single MAPI session. In the example above the MAPI session was trying to open more than the default 500 allowed objects of type “objtFolder” (in other words, Folders).

The solution is provided by Microsoft in this support article, which refers to Exchange Server 2003 but applies to later versions as well.

1. Open Regedit and navigate to \\HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
2. If the key “MaxObjsPerMapiSession” does not already exist, right-click on ParametersSystem and create a new key of that name.
3. Create a new DWORD value with a name matching the object type in the event log error on your server (eg “objtFolder” in this example)
4. Set the DWORD value to a decimal value higher than the default. For the case above I first tried 1000, then 10000 when that did not work.
5. Wait 15-30 minutes for the registry change to take effect, then retry the Outlook operation.

You may find that you need to try gradually increasing the value until you can successfully recover the items in Outlook. You may also find that after you’ve removed one limit that another object type’s limit may also suddenly come into play, so always check your event log for new errors identifying additional object types that you may need to add a registry value for.

After you’ve successfully recovered the Outlook items revert the registry keys to their default values (listed in the Microsoft support article above) to prevent issues with server resources in the future.