Dan,
While rare, it -is- possible for your host to restrict your messages based on the "Send From Email" needing to exist as a mailbox or forwarder. Even more rare, the "Send From Email" must match the email account used for authorization. Further restrictions might be in place on the "Return-path" (bounce to) address.
Have you tried without authorization? A hostname of "localhost" with no authorization could work.
The error message "Administrative Prohibition" could also be due to other reasons such as per-connection, hourly or daily limits on email. I recommend asking your host about the possibility of such limits.
Regards