It happened to me a couple of times, after the migration of public folders from an Exchange 2003 server to an Exchange Server 2010 mail enabled public folders would stop receiving emails following the complete disposal of the 2003 server (soon after its uninstallation).
All mail addressed to the public folders remained in the queue and were not delivered (you could see them from the queue viewer) despite the public folders existed and they had the correct alias. In some cases this error message back to the sender:
#< #5.2.0 smtp;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.> #SMTP#
If you find yourself in this situation you need to open an adsi edit (from the exchange server or from a domain controller), from the menu on the top action-> connect to and set “Configuration” as naming contest.
Now open the tree going into Services, Microsoft Exchange, First Organization, Administrative Groups, First administrative group (or the name of your old exchange organization)
If you find a folder “Servers” like the one you see in the screenshot check that inside there are no server and, If it is clean, delete it! The mails addressed to the public folders that have been queued will begin to flow and to be delivered.