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

[Michael Thomas <mike () mtcc com>]

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=

Mike