Re: [Asrg] Fallback to MX

John Levine <> Thu, 11 February 2010 15:38 UTC

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Date: 11 Feb 2010 15:39:55 -0000
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From: John Levine <>
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Subject: Re: [Asrg] Fallback to MX
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>I understand the spec. I've not read the discussions leading to it, but if 
>there's a GOOD reason to assume every AAAA record (without a corresponding 
>MX record) points to a mail host, then I'd like to hear it.

I'm with you.  When they were finishing up 5321, I made the obvious

* Fallback to A was a transition that should have been deprecated in 1990

* The vast majority of hosts with A or AAAA but no MX records are not
  mail domains

* Fallback to A makes mail less reliable, since mail sent to a domain
  with an A record on a non-responsive host will take a week to bounce

* The number of IPv6 mail hosts now is tiny, insignificant installed base

* Anyone moving from IPv4 to IPv6 will have to change the DNS anyway,
  so adding an MX record when you add an AAAA is not a big deal

Nonetheless, I lost, they said it was "too much of a change".  Dave was
there and might be able to elucidate.

There is no chance whatsoever that the RFC standards will change.

The idea of not accepting mail from domains with only AAAA records
might be worth pursuing. I agree that despite what 5321 says, it seems
pretty unlikely that such domains would ever send mail anyone wanted
to receive.