Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions

Michael Thomas <mike@mtcc.com> Wed, 25 November 2020 19:16 UTC

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From: Michael Thomas <mike@mtcc.com>
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Date: Wed, 25 Nov 2020 11:16:34 -0800
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Subject: Re: [dmarc-ietf] A policy for direct mail flows only, was ARC questions
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On 11/25/20 11:11 AM, Alessandro Vesely wrote:
> Hi,
>
> On 25/11/2020 19:24, Jesse Thompson wrote:
>> On 11/25/20 11:30 AM, Alessandro Vesely wrote:
>>> Without resorting to ARC, it is still possible to validate author 
>>> domain's signatures directly if the MLM just adds a subject tag and 
>>> a footer, like, for example, this list does.   While ARC solves 
>>> "deep" forwarding problems, which may arise in the context of email 
>>> address portability, MLM transformation reversion solves the simpler 
>>> mailing list problem, including reverting munged From:'s.
>>
>> I agree that ARC isn't really needed to do this (trust the last hop 
>> from the MLM and determine the original authenticity from the MLM's 
>> perspective)
>
>
> I didn't mean to trust the MLM.  I meant remove the subject tag and 
> the footer, then the original DKIM signature verifies.  See:
> https://datatracker.ietf.org/doc/draft-vesely-dmarc-mlm-transform/


When I was at Cisco, with l= and some subject line heuristics I could 
get probably like 90+% verification rate across the entire company, a 
company that uses external mailing lists a lot. Definitely not 100% though.

Mike