It is possible to move a mailbox from exchange 2007 to exchange 2003, but I do not know about restoring it. Does not sound likely because exchange 2007 has so many different attributes than exchange 2003 does. But you could restore to exchange 2007 and move the mailbox to 2003. I don’t know if that is the solution you are looking for though. Can you tell us why you would need to restore it to an exchange 2003 server?