I recently came across the following issue while performing an exchange 2007 to exchange 2010 migration for a company that still heavily utilised mail-enabled public folders. The client's Exchange environment was quite old and had been previously migrated from Exchange 5.5 to 2003 and hadn't been properly decommissioned with the original move to 2007, but that's another story!
The issue I had was that once I moved all receive connectors to the new Exchange 2010 server, emails sent to any of the mail-enabled public folders (which were still being replicated between the 2 servers) was being rejected with the following ndr:
The following error was found in Event Viewer:
Log Name: Application
Source: MSExchange Store Driver
Date: 7/09/2010 2:00:02 AM
Event ID: 1020
Task Category: MSExchangeStoreDriver
The store driver couldn't deliver the public folder replication message “Folder Content (PublicFolderDatabase@domain)” because the following error occurred: The active directory user wasn't found.
Given this Exchange environment was originally running versions prior to exchange 2007, the legacy Administrative group and all its empty subcontainers still existed. An explanation from the microsoft Exchange Team of why this issue occurs can be found here: http://msexchangeteam.com/archive/2010/05/05/454821.aspx
The solution to this issue was to delete the empty Servers container from the legacy Administrative Group.
Using ADSIEdit.msc, connect to a domain controller and navigate to:
CN=Configuration , CN=Services, CN=Microsoft Exchange, CN=[ExchangeOrganisationName], CN=Administrative Groups, CN=[LegacyAdministrativeGroupName], CN=Servers.
Right click the Servers container and select Delete
Restart the Microsoft Exchange Active Directory Topology Service and the Microsoft Exchange Information Store on both the Exchange 2007 & Exchange 2010 Servers