There are probably tools around to do this, but you can rebuild your OE folders manually. It sounds like you Outbox is corrupted. First off, create a new temporary folder(s) and put all the emails you don't want to lose in it. You can do it separately like Temp Outbox, Temp Inbox, etc, or just put all of them in one temp folder. You can move them back later.

Once you have them safely tucked away in another folder, close OE and go to your identity in OE. You may have to unhide files and folders. Find your identity in explorer and then look at the folders in the right pane. You can delete the Inbox, Outbox, Sent, Deleted and Drafts; all of them or just the Outbox if you'd rather do that. DO NOT delete the Folders.dbx.

Then next time you open OE, the folder(s) will be rebuilt.

Always make a system restore point first, just in case and/or save copies of those .dbx files before deleteing them. But I've done this many times with no problem.

Hope this helps.

This is also a good way to clean out OE when backups become too large.