A fix as been released by M$ to resolve the problem of undeletable mails:

"Messages may be stuck in the Outbox when you use Windows Mail on a computer
that is running Windows Vista"

http://support.microsoft.com/default.aspx/kb/941090

Ignore the title, it works on other folders in addition to the Outbox.

não consigo deletar uma mensagem na caixa de saída
o que fazer