lists and DMARC and ARC, was Incoming SMTP in the year 2017 and absence of DKIM

Michael Thomas mike at
Wed Nov 29 23:03:17 UTC 2017

On 11/29/2017 03:00 PM, Grant Taylor via NANOG wrote:
> On 11/29/2017 03:46 PM, Michael Thomas wrote:
>> You know what the original header was via the signature. You can take 
>> the delta of the current subject line and remove any additions and 
>> validate the signature. Whether you're happy with the additions is a 
>> different concern,
> Are you referring to the optional z DKIM-Signature tag?
> Or are you referring to brute forcing what the subject was in order to 
> derive the delta of the current subject?  This would be compounded by 
> any number of other changes to (over)singed headers / body portion.
>> If I were constructing a spam filter out of it, I'd give a lot of 
>> prejudice to anything added, but that's outside of
>> what you can do within the bounds of the spec.
> *If* the z tag was included in the DKIM-Signature header, I can see 
> how this would work and I agree with your distrust of said additions / 
> alterations.

Yes, with the z=


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