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Re: How does SMTP IPv4 and IPv6 work together

2008-04-05 22:34:13

At 10:25 -0400 on 04/05/2008, Hector Santos wrote about How does SMTP IPv4 and IPv6 work together:

To me, this all makes common sense.  I think the text in 2821bis should
simply lay down the possibilities as done above.  But I do think the
above can be folded to make it into smaller text.

While I agree 100% with your CONNECTIVITY charts/etc. You are ignoring the question of WHEN/IF AAAA records should/can be used in the absence of EXPLICATE MX that points at the AAAA via its RHS. We have to support IMPLICATE MX creation pointing at A records for the FQDN RHS of the email address due to a refusal to enforce the "Mail Servers are pointed to by EXPLICATE MX records" requirement that should have gone into effect at least 10-15 years ago once use of A records to find Mail Servers was superceded by the definition of MX records. While we still support IMPLICATE MX records pointing at A records, it is just to support lazy administrators who not bother to define MX records or who run MTA software without MX support or with crippled MX support. I am NOT saying to drop IMPLICATE A-Record MX generation but only to not go down the "AAAA without MX is supported" route. This is NOT the 1980s where we need to support MX-less FQDNs with A records but 2008 when there is NO VALID REASON to run an IPv6 MTA that is not pointed to by an EXPLICATE MX. IMO: Any IPv6 MTA that expects to receive email is misconfigured if it is not pointed to by an EXPLICATE MX.

What justification do you offer to allow an IPv6 MTA to run without an EXPLICATE MX record? The fact that we allow IPv4 MTAs to run with only an A record is NOT a valid excuse since that was/is permitted due to A-Usage predating the existence of the MX record and thus allowed (although that allowance should have been rescinded years ago).