Vista - Shutting down Windows Mail

When I shutdown Mail the window closes. However, when I try to reopen Mail
during the same session the following legend appears: 'Windows Mail could not
be started because another instance of it is still running on your computer.
Please log off of Windows, and then restart Windows Mail. If the problem
persists, save your work, and then restart your computer.'
Despite logging off, restarting mail/ computer, the problem still persists.
I exit Mail through File/Exit but still the process 'winmail.exe appears on
the Task Manager.

Can anybody help ?