On Tue, Dec 21, 2021 at 11:32 AM John Levine <jo...@taugh.com> wrote:
> The DNS has had a formal definition of non-existence for over 30 > years. You look up a name, if it returns records or NOERROR it exists, > if it returns NXDOMAIN it doesn't. There is no reason for us to invent > something new and incompatible. > > >I don't remember exactly why we settled on A/ AAAA/ MX, but the lack of a > clear, actionable definition is why we included one. > > See above. I don't remember where the text in A.4 came from, but it is > wrong. > If we are telling people to test whether a domain exists, they should do it > the way the DNS does it. The correct test happens to be cheaper than A.4, > one query rather than three. > We're talking about two different things here, I think. The DNS definition of "nonexistent" is as cited above while the DMARC definition matches the well established SMTP algorithm that figures out where the next hop for a particular recipient is. If there is no next hop, then for email purposes, the domain doesn't "exist". The logic goes something like: If this message fails DMARC, and a bounce has nowhere to go, then I'm pretty sure I don't want to deliver it. We're free to change our minds about what test is appropriate here, but that was the genesis of A.4. -MSK
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