Scott Kitterman wrote:

> Who adds the ARC stamp? Perhaps I read it wrong, but I read it as being 
> added by the intermediary and not the originator (previous hop).

That's correct.

> If I read it right, anyone can create an ARC stamp claiming to have 
> received authenticate (e.g. DKIM signed) mail from my domain.  

Correct, but unlike a received header, that bad actor has identified themselves 
by way of their signature on the claim. Having this stronger identification of 
the intermediaries is a key feature of ARC. 

By itself though the identification is not enough - it doesn't tell the 
receiver that the claim is false; the receiver must independently assess the 
trustworthiness of each ARC intermediary, by way of a reputation system or 
otherwise. The hope is that having a strong and automated way to identify the 
intermediaries will make creation and maintenance of the reputation system 
simpler, and increase its accuracy.

So in the end the receiver is holding a message, which by content analysis or 
otherwise it classifies for delivery. If the content classification is strongly 
negative not even passing DMARC is intended to override the classification (and 
certainly ARC results on a failing DMARC shouldn't). But that negative 
classification can drive the reputation engine. With a weak content 
classification result the ARC chain evaluation MAY be used to guide the local 
decision, leading to an override of the p=reject or not.

-- Shal

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