Re: [dmarc-ietf] ARC vs reject

Alessandro Vesely <> Mon, 07 December 2020 10:31 UTC

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From: Alessandro Vesely <>
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Date: Mon, 7 Dec 2020 11:31:31 +0100
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Subject: Re: [dmarc-ietf] ARC vs reject
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On Sun 06/Dec/2020 05:14:18 +0100 John R Levine wrote:
> On Sat, 5 Dec 2020, Jim Fenton wrote:
>> ... If the recipient domain accepts modifications by zero-reputation 
>> intermediaries (because there are so many of them, after all)
> I wouldn't call that a reasonable implementation of ARC.  The set of hosts that 
> are likely to send you mail with interesting ARC chains is relatively small, 
> and I don't think it changes very fast.

Trustworthiness has to account for the probability that a trusted host is 
hacked, even occasionally, so as to spew phishing.  Reasonableness is a number 
in [0, 1].  In the presence of a chain, one must consider the joint probability 
that any intermediary is hacked.

Anyone observed long ARC chains?

> I'd certainly be interested in hearing how people plan to compile and maintain 
> their lists of ARC-worthy hosts.

There should be a means of exchanging trustworthiness values, so as to build 
the transitivity required to compute the joint probabilities.