Originally Posted by capabel

Do you use exchange or a third party ISP and pop3 e-mail? If your using an ISP just delete it from the server.

I've seen a corrupt ost cause this in the past as the e-mail flag never shows as "read" so outlook thinks it has a new e-mail it needs to reply to on every send / receive. Even after a scan it would still do it so I was able to fix that and a few other odd problems by creating a new ost and moving all the data to the new file.