At 18:00 -0400 on 04/30/2008, John Leslie wrote about Re: Queued Mail
or Unreturnable Mail?:
We certainly could design a mechanism _better_ than MX records to
document intent relative to receiving email, especially DSNs. But unless
and until we do, folks are likely to use MX records as part of the
balancing act of guessing the probability of NDNs reaching a responsible
reader.
IMO: The publication of a MX is a statement to "Send *(_at_)FQDN Addressed
Email HERE". Once an SMTP Server does so and delivers a NDN message
addressed to *(_at_)FQDN to the MX, it has done its job and turned over
Delivery Responsibility to a MX designated Server. The fact that the
MX Server (or one that it forwards the NDN message to) then turns
around and sends is to /dev/null is not my SMTP Server's Problem
since, as noted, it did its job by delivering the message to the
Domain Designated injection point and turning over further delivery
processing to the Domain's Mail System.
Thus the existence of the MX places the responsibility of "reaching a
responsible reader" on the MX not the Sending SMTP Server with NO
GUESSING involved.