Ned, by "disable MX lookups", do you mean "don't put MX records
into the DNS zone and therefore force a fallback to the address
records" or "ignore the requirement of the standard that
requires using MX records if they are there"? If the latter,
the behavior, however useful (or not) is, IMO, so far outside
the standard that it is irrelevant to any discussion about how
DNS records are used in a standard way.
Keith, what do administrative domains have to do with this?
I can imagine wanting an option to skip MX record lookup for relaying
mail within an administrative domain - if, say, all of the relevant
domains within that administrative domain were known to have A records
pointing to their inbound SMTP server.
By contrast, I have a hard time imagining a good reason to skip MX
lookup for relaying mail from one administrative domain to another.
(unless maybe there is a bilateral agreement between the source and
destination domain that specifies how such mail is to be relayed.)
(I have no doubt at all that people ask for such a thing, but I am at a
loss as to why such behavior should be blessed by the standards.)
IETF mailing list