Not necessarily. That host may apply a different policy for mail
coming through the other MX hosts. Otherwise, it would cause
backscatter. BTW, the problem may be applicable to IPv4 as well.
Oh wait, I misunderstood what you're saying. And you probably misunderstood me.
jck.com doesn't have an IPv6 MX, so the only way to deliver mail to it
is to open an IPv4 SMTP connection.
Suppose I have only an IPv6 address and want to send John mail (sorry to
pick on you, John). I open an IPv6 connection to my ISP's smarthost and
send some mail from my address to a jck.com address. My ISP's smarthost
has an IPv4 address, so it opens an IPv4 connection to the best
reachable MX for jck.com. The connection from my ISP's smarthost to the
jck.com MX is what I pasted, and where the problem shows up.
And the text I think should be there would be a sentence like this in
sections 220.127.116.11: "Note that the reverse-path may be reachable only via
IPv6, even if the SMTP connection uses IPv4", possibly with some
emphasis to point out that perfectly valid 2821 code/configurations
don't support this. 18.104.22.168 would need similar language for