Once you have moved all of your mailboxes to Exchange 2010 and properly decommission your Exchange 2003 (move public folder hierarchies, transition mail flow, move OAB generation, etc.), you may see MSExchange Store Driver event ID 1020 errors like these:
The store driver couldnt deliver the public folder replication message “Hierarchy (PublicFolder@client.com)” because the following error occurred: The Active Directory user wasn’t found.
These errors likely coincide with some public folder replication issues but, more importantly, can also result in NDRs when messages are sent to mail-enabled public folders! This issue occurs because, even if you properly decommission Exchange 2003, the Servers containers within your legacy Exchange 2003 Administrative Groups still exist within Active Directory, albeit empty. Exchange assumes that, if a Servers container exists (even if it is empty), a System Attendant object will also exist somewhere inside of it but, if all of your Exchange 2003 servers have been decommissioned, those System Attendant objects actually do not exist.
This issue has been recognized as a bug within Exchange (see the MS Exchange Team Blog article here) and a fix a scheduled for an upcoming Update Rollup release (perhaps Update Rollup 5). In the meantime, you can safely delete these empty Servers containers via ADSI Edit but make sure that these containers completely empty before doing so.
For more in my series on Exchange 2010 Notes from the Field, please click here.