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
