I think that you need to get into your in-box on the server and then delete the stuck messages. If your ISP is Freeserve (or whatever it's called now) it's easy. Go to the Member Centre and sign up for FSmail (it's free). This is a web-based service like Hotmail. You can the go directly to the in-box and delete as you wish. If you are with a different ISP they might have a similar arrangement.