[nh2]

This doesn't sound perfectly accurate for my case (since the DMARC policy seems to be for paypal.co.uk, not .com), but quite related / perhaps the other way around?

I see in the headers:

    ARC-Authentication-Results: i=1; mx.google.com;
       dkim=pass header.i=@paypal.co.uk header.s=pp-dkim1 header.b=UsIpWUs9;
       spf=pass (google.com: domain of [e-mail] designates 173.0.84.226 as permitted sender) smtp.mailfrom=[e-mail];
       dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=paypal.co.uk
    Received: from mx0.slc.paypal.com (mx1.slc.paypal.com. [173.0.84.226])
            by mx.google.com with ESMTPS id q195si2496022ita.120.2019.02.05.13.10.37

> this is why you see the via in the from

I am not sure, it may also appear due to what I said in https://news.ycombinator.com/item?id=19100315