On Wed, Aug 16, 2017 at 5:47 PM, Bron Gondwana <br...@fastmailteam.com> wrote: > > While there exists A SINGLE SITE which is ARC-unaware and DMARC p=reject > aware, you can't use ARC on a DMARC p=reject domain without rewriting the > sender. Otherwise that site will bounce the email. > > You still have to rewrite the sender until there is either full adoption > or sufficient adoption that you just don't care about the stragglers. > > ARC doesn't solve that unless every receiver is either ARC-aware or > DMARC-unaware. Hence the suggestion that I think Hector is making - to > define a new policy p=rejectnonarc or something, which means that sites > which are ARC-unaware accept rather than reject. >
So this is an excellent and crucial point that has been discussed again and again on and off this list. ARC only works if critical mass can be reached - both of intermediaries who break DKIM and receivers who evaluate ARC. Fundamentally, ARC will never reach critical mass if senders now also need to enter into the equation with an additional flag on their DMARC record. This is too high a bar and increases the interoperability problems as opposed to reducing them. For now, there is a clear unanswered question: what coverage of receivers is needed for most mailing lists to achieve stable delivery once ARC is in the mix? Knowing the landscape of receiving domains, I believe this to be a small number. From the above comments, I'm guessing you think this is a large number. We're not going to resolve this difference of opinion in an open forum. However, releasing ARC as an experiment into the wild for major lists like the IETF and M3AAWG will give us very clear data very quickly on what the actual landscape looks like, and what ARC does and does not solve. In its current form, ARC only helps mail flows, it does not harm them. How effective this improvement is remains to be seen, but preliminary information I've been hearing about (which could be totally wrong) makes it seem like the improvements are dramatic. So let's get ARC tied off as an Experiment (thank you, Dave Crocker), collect some data, and see where things stand. Maybe things are great and ARC can move to proposed standard. Maybe it fundamentally needs more receivers in the mix than currently expected, and some fix for that is needed. But we'll know that after the experiment has begun, not before.
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