As anyone who uses MSMail with Outlook 2002 knows, Microsoft dropped support for MS Mail from Outlook 2002. Outlook MVPs Vince, Dmitry, and myself worked out how to restore MS Mail to Outlook 2002 and we now have the steps needed for Outlook 2003. Changes include: mapisvc.inf is now stored in a new folder and you need two MSFS*.ecf files, along with the MSFS32.dll required for Outlook 2002. The Post Office address book doesn't work (you can view addresses but not send to them), however if you add the members of your post office to Contacts you can send them mail. Replies and addresses stored in autocomplete work fine too.
Instructions for Outlook 2003 are now at http://www.outlook-tips.net/msmail.htm.