Many using Windows Mail in Windows Vista might have faced the problem of odd messages or mails being stuck in Outbox of Windows Mail and nothing can be done about it. You can't send , can't delete or even can't access it, although it shows in Outbox. Additionally, every time that you close Windows Mail, you are prompted that there are unsent messages in your Outbox.
This problem occurs when the stream entries for the messages are inaccessible before Windows Mail deletes the messages from the Outbox.
Now a Fix has been made available for it. To resolve this problem, apply this update. After you apply the update, you can delete the messages that are stuck in the Outbox.
Note Because the messages are permanently deleted, you have to resend the messages.
Goto the following link and download the update.
Messages may be stuck in the Outbox when you use Windows Mail on a computer that is running Windows Vista(KB941090)
This update applies ONLY to Windows Mail in Windows Vista.
Some people may also be facing this problem using Windows Live Mail (beta) in Windows Vista or Windows XP, that I think will be taken care in the newer builds of beta as it moves towards the final build and out of beta.