ADSelfService: trying to send any email generates "Invalid Email-ID" message.

ADSelfService: trying to send any email generates "Invalid Email-ID" message.

What IS and "Email-ID"? Why do I get this error? I've tried sending a test email from the configuration screens, as well as sending out enrollment notification emails, and I get this error on every attempt.
              New to ADManager Plus?

                New to ADSelfService Plus?