At 14:29 -0500 on 04/08/2008, Pete Resnick wrote about Re: What is
the history of 2821 and implict MX?:
The MX rule of 974 is perfectly reasonable to apply to IPv4 and IPv6.
RFC 974 also requires the use of a WKS record to verify that the IPN
address has a MTA running on it. Since we do not use WKS records for
this purpose, I'd suggest that for IPv4 only MTAs (handling a FQDN)
the RFC 974 rule of the MX being optional be observed BUT once FQDN
maps to an AAAA record (or both AAAA and A) that the MX be required
in the case where all the IPNs that are mapped to FQDN are NOT
running a MTA so that the RHS can supply a FQDN1 other than FQDN
(IOW: Use the list of RHS FQDNs to supply the function that WKS would
have supplied). Thus the MX is still optional if all IPNs mapped to
the FQDN are running a MTA.
IMO: The implicit MX (ie: Generating a FQDN MX 0 FQDN) per RFC 974 is
ONLY valid if every host that is mapped via the FQDN IN is a MTA. So
long ANY are not running a MTA then the lack of a MX that points ONLY
at MTAs is a definition problem (since non-MTA running IPNs in the
FQDN IN list can be erroneously contacted to try to pass them relayed
Note that this is a change in my prior "MUST have MX for AAAA
records" stance. I now would like to see EITHER a MUST have an MX if
there is a IPv6 Server requirement OR an EXPLICIT statement that the
MX is optional only when all IPNs are running a MTA.