Try the fix provided here:

Messages may be stuck in the Outbox when you use Windows Mail on a computer
that is running Windows Vista
http://support.microsoft.com/kb/941090

The article instructs you to download and install the update appropriate for
your operating system, found here:

Windows Vista, 32-bit versions:
http://www.microsoft.com/downloads/d...5-A86F2B7E39F1

Or,

Windows Vista, 64-bit versions:
http://www.microsoft.com/downloads/d...C-BE5ABE3F39B7


"Ange" <Ange@discussions.microsoft.com> wrote in message
news:67B60886-1B0B-4F91-88B6-05E9274A89E3@microsoft.com...
> Since 21st August, I have been unable to send e-mails although I can
> receive
> them without a problem. I had a similar problem in March when Microsoft
> advised me to restore my system to a date prior to a recent update. This
> worked and up until 21st August there have been no problems. My husband
> has
> exactly the same outgoing mail set-up and can send e-mails without a
> problem.
> I have restored my system to 17th August but still no joy. There is an
> e-mail in my outbox that I cannot delete which I suspect is causing the
> problem. Any advice?
> --
> Ange