Try Microsoft's 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"
That page offers two download options. Unless you know for sure
you have 64-bit Vista, use the 32-bit option.
--
Gary VanderMolen [MS-MVP WLM]
"Hamish" <> wrote in message news:199CDD5A-B2EB-4911-B9A1-...
> got a message stuck in the outbox - I cant delete or modify it. Body of text
> in the offending mail says
> Message could not be displayed
> Windows Mail encountered an unexpected problem while displaying this
> message. Check your computer for low memory or low disk space and try again.