Use the Microsoft fix:
http://support.microsoft.com/default.aspx/kb/941090
"Messages may be stuck in the Outbox when you use Windows Mail on a
computer that is running Windows Vista"
Although Outbox is the only folder mentioned, in fact it works with all folders.
Gary VanderMolen
"rlc75" <> wrote in message news:027AC489-5A24-4E8C-A02D-...
> I have one email in my inbox that every time I try to delete it I receive a
> "unknown error has occurred warning" .. I have emptied my deleted files box
> and tried again and it still will not delete. Any suggestions would be
> helpful.