You may have a stuck message in your Outbox. Do you receive any error messages?
What antivirus program are you using?
Give this a try to see if it will help you:
Read the section titled "FIRST" in this tutorial about antivirus and anti-spyware/adware programs to help prevent this from happening again, then install the KB941090 Microsoft Update in the section titled "SECOND" to fix and be able to delete the stuck message from the problem folder.