Messages may be stuck in the Outbox when you use Windows Mail on a computer that is running Windows Vista
On a computer that is running Windows Vista, some messages may be stuck in the Outbox when you use Windows Mail. You cannot send or delete these messages. Additionally, every time that you close Windows Mail, you are prompted that there are unsent messages in your Outbox.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB941090.