After a Windows Update, Mail Enabled Public Folders produce the following NDR

#554 5.2.0 STOREDRV.Deliver.Exception:ObjectNotFoundException; Failed to process message due to a permanent exception with message The Active Directory user wasn’t found. ObjectNotFoundException: The Active Directory user wasn’t found. ##

To Fix this do the following :

Go to adsiedit.msc.

Locate: CN=Configuration\services\Microsoft Exchange\Administrative Groups\Name of Legacy Admin Group\Servers

Check that the “Servers” container is empty. If so, right-click on “Servers” and delete the container.