Toshiba computers often come with a program with SMS in its
name already installed, even though this program causes
problems in Windows Mail.  Check your list of installed
programs to see which programs with such names you have
installed, and list them.

Also, some antivirus programs also cause problems in
Windows Mail.  What if any antivirus programs do you
have, and have you ever had a Norton or McAfee
antivirus program on that computer?

Click on the Outbox, then each of the messages in it.
If any show an error message instead of a message
you tried to send, then for a temporary fix of your
problem, 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.

If you still have a problem and get an error message when
trying to send a short message to yourself, right click on
the error message and copy all of it to your next message.