Dave Crocker wrote:
the change moves the MX from being an administrative
convenience to instead being a core requirement, ie,
barrier to deployment.
As far as the wording "implicit MX" means something the
"MX" concept is not only an administrative convenvenience,
but fundamental. If a paradigm changed it was in RFC 974.
The "administrative convenience" was to make this not
explicitly, when it is in a way redundant for a single
IPv4 host acting as its own "implicit" MX.
For the transition to IPv6 many hosts limited to IPvX
will have to arrange for an explicit MX, handling any
non-IPvX mail for them.
How many hosts will have both A and AAAA for the time
of the transition, and consider adding an explicit MX
as deployment barrier ?
Frank