John C Klensin writes:
One observation, having worked through this several times before
realizing that the answer is obvious and feeling a need to be sure
that we are in agreement about what we are discussing...
We agree about almost everything. Our only disagreement is about whether
AAAA records should influence the decision whether to synthesise an MX
Consider a case where 2821 and 2821bis gives a different result: An
IPv6-only network printer. Most of my printers can send mail, none
listen on port 25, and when the IPv4 network becomes tight, the
printers will be among the first devices to lose their A RRs, so this
is a good example.
Code implementing the rules in 2821 will correctly decide that mail to
the network printer's host name isn't deliverable, code implementing
the rules in 2821bis-06 will infer an MX.
2821: An MTA being told to send mail to the printer rejects at once.
2821bis-06: The MTA tries for a few days, then gives up.
2821: A web form rejects fadsfs(_at_)printer(_dot_)example(_dot_)com as bad
2821bis-06: A web form accepts fadsfs(_at_)printer(_dot_)example(_dot_)com as a valid
Similar for the other uses and devices I thought of: 2821 code makes the
practically correct decision, 2821bis-06 the wrong one.
(I deleted the rest of what I quoted, because my responses were just yes