Microsoft has a fix for that:
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]
"Bjames" <(E-Mail Removed)> wrote in message news

B12F144-6896-495B-ABB5-(E-Mail Removed)...
> I'm a dummy, and I can't send a message until I get this deleted from my
> outbox. When I hit delete, it tells me there's an error. I've already turned
> the computer off and unattached the modem. Restarted w/out the modem and hit
> delete, but it did not help. Help me!