Re: [dmarc-ietf] ARC questions

Doug Foster <> Mon, 23 November 2020 17:15 UTC

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From: "Doug Foster" <>
To: "'Dave Crocker'" <>, "'Todd Herr'" <>, "'Joseph Brennan'" <>
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Date: Mon, 23 Nov 2020 12:15:26 -0500
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Subject: Re: [dmarc-ietf] ARC questions
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My wishlist for ARC:


ARC tells me that somebody changed some data, but it does not tell me which MTA performed the forwarding operation, added content, or performed address rewriting.  If we could get HELO names into the ARC data, then those names could be correlated with the Received header chain to make better filtering decisions.




From: dmarc [] On Behalf Of Dave Crocker
Sent: Monday, November 23, 2020 12:02 PM
To: Todd Herr; Joseph Brennan
Subject: Re: [dmarc-ietf] ARC questions


On 11/23/2020 7:38 AM, Todd Herr wrote:

On Mon, Nov 23, 2020 at 9:50 AM Joseph Brennan <> wrote:

On Sat, Nov 21, 2020 at 7:14 PM John Levine <> wrote:


This also means that ARC isn't useful if you don't have a reputation
system to tell you where the lists and other forwarders that might add
legit ARC signatures are. 


And if you know which hosts are legit mailing lists or forwarders, you already know what ARC would tell you.


I believe, though, that the intent of ARC is that it be scalable in ways that manual enumeration of known legit mailing lists and forwarders is not. 


"if you know which hosts are legit" buries an assumption that is problematic, namely that you know who handled the message.  The fack that a message purports to be handled by a mailing list you trust does not mean it actually was.

That's the issue that ARC resolves.

ARC (and DKIM) produce noise-free uses of identifiers.  If the authentication validates, the receiver knows is really was handled by who is saying it was handled by.  Without these, you don't.



Dave Crocker
Volunteer, Silicon Valley Chapter
American Red Cross