Click on the Outbox and then each of the messages there.  If any
display an error message instead of a message you tried to send,
move them to the Drafts folder or delete them.  If they 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 messages 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.
The Vista SP1 update includes this hotfix and more, but takes longer to
install.

Some antivirus programs occasionally cause stuck message problems.
Therefore, let us know what antivirus programs you have, and whether
you have ever had a Norton or McAfee antivirus program on that
computer, so we can tell you more about making the fix more permanent.