John R Levine: > >> No, 550 to reject the MAIL FROM is correct. See RFC 5321, sec 3.3. It's > > > > My concern is with sending 521 after RCPT TO. Sorry for coming > > with this late in the process. I wasn't aware of the nullmx revival. > > If you get 521 as a server greeting it means ... Thanks, there was no need to repeat that another time. I infer from your response that you are not concerned about consistency between 4XY and 5XY SMTP responses, not even when until now all X21 SMTP responses end a session whereas the proposed 521 does not. And with that, I will leave this discussion. Wietse