Re: [dmarc-ietf] ARC vs reject

Alessandro Vesely <> Sun, 06 December 2020 18:31 UTC

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From: Alessandro Vesely <>
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Date: Sun, 6 Dec 2020 19:31:30 +0100
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Subject: Re: [dmarc-ietf] ARC vs reject
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On Sun 06/Dec/2020 18:01:04 +0100 Michael Thomas wrote:
> On 12/6/20 5:40 AM, Alessandro Vesely wrote:
>> On Sun 06/Dec/2020 02:34:45 +0100 Michael Thomas wrote:
>>>> 5) The work you and Alessandro have done with reverse transformation is 
>>>> more likely to produce a solution for the mailing lists.   The lists will 
>>>> continue to do From rewrite, but reverse-transform recipients can validate 
>>>> the true source of the message and restore the From if desired.
>>> I'm starting to get a little more serious about my quip that the MLM can 
>>> insert a sed script in a header to unmangle the message since it knows what 
>>> transforms it has done, unlike the receiving MTA trying to guess the common 
>>> transformations.
>> But then the receiving MTA will have to guess whether the sed script 
>> considerably alters the intended meaning of the message. For example, does it 
>> change a bank account number?
> This actually highlights why my observation is correct. If the intermediary 
> showed how to reverse their changes perfectly to be able to validate the 
> original signature, it says nothing about whether those changes to be delivered 
> to the recipient are acceptable to the originating domain. for the case of a 
> bank sending me sensitive mail, the answer is that it is never ok. for somebody 
> working on internet standards working on ietf lists, the answer is that it is 
> fine. hence trying to get two states of the one "reject" is insufficient.

For MLM transformations, this choice can be done by tuning DKIM signatures.  A 
bank can choose to sign Sender: field (or lack thereof), or any other fields 
that a MLM has to change, and possibly use simple canonicalization.  In that 
conditions, transformation reversion won't work.  It isn't a distinct DMARC 
state, formally.  Yet, tuning DKIM signatures allows to harden or weaken the 
given DMARC state.