We are in the process of migrating a client to Exchange 2010 from 2003 - all looks good. We have however one error. On mailboxes we've move to the Exchange 2010, when we try and download the OAB in Outlook 2003 we get an error - "An error occurred whilst opening the Microsoft Exchange offline address book files......". The OAB is hosted on the Exch 2003 server and a replica is on the 2010 server. Do we need to move the OAB to the 2010 server and upgrade it? I'm holding off doing this on the basis it may affect the users on the 2003 server?