A Winter said:
Have a message stuck in Outbox. Using Windows Vista.
The message will not send, and will not delete.
Any new messages after this will not send.
Windows mail says unknown error occured.
Download and install this fix, with Windows Mail not running:
http://support.microsoft.com/default.aspx/kb/941090
Although it's labelled as for mail stuck in the Outbox, it 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. Certain common
antivirus programs, such as most of those from Norton or McAfee,
or a recent one from Trend, tend to cause such problems, so if you
have such a program, remember where to find the fix - you are likely
to need it again.