Keith Moore writes:
the IPv4- or IPv6-only MTA may wish to employ one or more external
"last resort" relays which can deliver to the other kind of MTA.
Under the RFC 2893 transition plan, hosts with IPv4 addresses can talk
to hosts with IPv6 addresses, so these SMTP relays are unnecessary.
Which makes more sense: configuring separate IPv6 relays for every
application, or configuring one relay for the network stack?
Apparently there is some dispute about the RFC 2893 transition plan. Are
IPv4 addresses going to be able to talk to IPv6 addresses, or is every
existing Internet site going to have to acquire IPv6 addresses?
I object to imposing IPv6 transition requirements on SMTP, or on any
other application, until these fundamental issues have been settled. I
am amazed that this group has not drawn a coherent picture of how the
IPv6 transition is supposed to work.