John Leslie wrote:
Minimizing the change _does_ tend to ease implementation...
and particularly should not make any changes to its registration model.
That really doesn't follow.
In particular, there are a number of possible gateway-like options
for communication between an IPv4-only host and an IPv6-only host.
It might be _very_ helpful to register preferences on which to use.
The registration model is not about v4 vs. v6. It is about use of A (or AAAA)
vs. MX in order to find an SMTP receiving host.
And the problems with requiring MX registration, in order to receive mail, have
now been well documented over the last few days.