In Windows Mail, unknown errors and undeletable messages are usually due to
interference from an antivirus program. What if any antivirus programs do
you
have?
For a temporary fix, download and install this hotfix, with Windows Mail not
running:
Although it's labelled as for stuck messages 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 also includes this hotfix, but takes longer to install.