Click on the Outbox, then move any messages you find there to the
Drafts folder or delete them. If any refuse to do either, download and
install this hotfix, with Windows Mail not running:
http://support.microsoft.com/default.aspx/kb/941090
Although it's labelled as for mail stuck in the Outbox, it also works in
other folders. If you don't know whether you have a 32-bit version of
Vista or a 64-bit version, try the hotfix for the 32-bit version first.
Some common antivirus programs, such as most of those from Norton
or McAfee, tend to cause stuck messages and other problems in
Windows Mail, although seldom immediately, so if you have such a
program, remember where to find the hotfix and hope it will install
again if you need it again. A recent version of the Trend antivirus
program also causes problems in Windows Mail.
The Vista SP1 update includes this hotfix and more, but takes longer
to install.