On Mon, 21 Dec 2009, John C Klensin wrote:
If implicit MXs continue to be permitted, this proposal, as I understand
it, would not work.
I believe it will work. RFC 5321 explains it twice:
If an empty list of MXs is returned, the address is treated as if it
was associated with an implicit MX RR, with a preference of 0, pointing
to that host. If MX records are present, but none of them are usable,
or the implicit MX is unusable, this situation MUST be reported as an
error.
If one or more MX RRs are found for a given name, SMTP systems MUST
NOT utilize any address RRs associated with that name unless they are
located using the MX RRs; the "implicit MX" rule above applies only
if there are no MX records present. If MX records are present, but
none of them are usable, this situation MUST be reported as an error.
Tony.
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