I have not run into this problem but have had a few that did not appear reparable. The only solution has been to remove SO totally. Re-install just as it was originally installed. Then replace backup copies of the database for the email. This has recovered all inbox, outbox, etc. messages. Before doing it, though, make sure you have a good backup copy of the data in another folder that is safe from SO messing with it. Place it on another machine, cd, or whatever location you can other than where it has been. If you installed from the cd you might try the repair feature, after backing up the data, but since my install of 5.2 was from the downloaded file repair has not worked. It requires you to install the cd. I have SO on several machines on a small lan and have the outbox of each set to copy any messages sent to an shared archive folder. By doing this I can see the sent messages from any machine on the lan and also don't have to worry about SO giving up. My outbox copy then becomes redundant.
James
James