When you are doing a migration from Exchange 2000/2003 to Exchange 2007/2010 you noticed that the OWA address is <servername>/owa. But when you try to connect to a mailbox still standing on Exchange 2000/2003 (not migrated yet) you are likely to receive the following error :
The solution is to user /exchange. Exchange 2000/2003 users will still be able to user OWA en Exchange 2007/2010 users are automatically redirected to /owa.