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 "I'm not a mail server." If
you get 521 as a response to RCPT TO it means "That's not a mail domain."
It's hard to imagine a situation where you'd want to retry either. It is
true that in the former case the whole session fails, while in the latter
case the recipient address is invalid, but those don't seem hard to tell
apart.
We're working on an 1846-bis that explains this more clearly.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
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