-** Run a program that hooks into some other system. The first argument is
-** name of the hook to use, the second is the full path name of a mail message.
-** The third argument is also the full path name of a mail message, or a NULL
-** pointer if it isn't needed. Look in the context for an error message if
-** something goes wrong; there is a built-in message in case one isn't
-** specified. Only produce the error message once.