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