FIX: Windows Mail - email stuck in outbox
Two updates have been released, one for Vista and one for Vista x64 - both require validation.
This update apparently resolves an issue where messages become stuck in the Outbox and cannot be deleted when using Windows Mail. After you install this update, you may have to restart your computer.
Vista
http://www.microsoft.com/downloads/details.aspx?FamilyID=09f002d9-a140-42a8-99f5-a86f2b7e39f1&DisplayLang=en
Vista x64
http://www.microsoft.com/downloads/details.aspx?FamilyID=09dabf39-1e37-46dd-91bc-be5abe3f39b7&DisplayLang=en