Sadly, though it is very late in the process, I failed to notice this originally, and must belatedly report a significant issue noted by Wietse Venema. The response code for rejecting a recipient with a nullmx domain and a sender with a nullmx domain are reversed in the draft. Since 521 like 221 and 421 leads to connection drop after the reply,
Only when it's the SMTP greeting. In this case it's not. That suggests that JCK's suggestion to have a new RFC to replace 1846 is a good one, since it could mention this other fairly obvious use case.
it is only appropriate when the entire envelope will be rejected. Thus 521 goes with rejection of a nullmx sender domain and 550 with a particular nullmx recipient.
No, 550 to reject the MAIL FROM is correct. See RFC 5321, sec 3.3. It's a policy rejection.
Regards, John Levine, johnl@xxxxxxxxx, Taughannock Networks, Trumansburg NY Please consider the environment before reading this e-mail.