I understand that some people might facing the problem as some emails stuck in the Outbox for Windows Mail. You cannot delete or move it to other folders. 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.
Microsoft already released the patch, please download the patch here.