It sometimes helps to make Windows Mail try harder to send the
messages. Click on Tools, then Options, then Send. If there's
no checkmark in the box before Send messages immediately,
click on this box, then Apply. Click on OK.
Also, if you still have messages in your Outbox after you restart
Windows Mail, try to move them to your Drafts folder or delete
them. If you find any that 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. Certain
common
antivirus programs, such as most of those from Norton or McAfee, tend
to cause stuck messages and various 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 assorted
problems in Windows Mail.
If you get error messages when you try to send or receive, after trying
the above fixes, click on them and try to copy the entire error messages
to your next message. Also tell us what if any antivirus programs you
have.