For a temporary fix, download and install this with Windows Mail not
running:
http://support.microsoft.com/default.aspx/kb/941090
It's labelled as for mail stuck in the Outbox, but also works in
other folders. If you don't know whether to use the 32-bit version
or the 64-bit version, try the 32-bit version first.
If you have one of the antivirus programs listed below, though,
you'll need to uninstall it, not just disable it, for a permanent fix.
You may even need to do more cleanup. For many other
antivirus programs, just leaving email scanning disabled is
enough.
Also, it sometimes helps to click on Tools, then Options, then
Send. If there is no check mark in the box before Send messages
immediately, click on this box and then Apply. Click on OK.