I will ask why the recipient system should look up anything but the dmarc record for the specific domain in the Header From.
In some cases looking up related domains is useful, and in some cases it can lead to disruption. We don't look up SPF records for related domains, because they are deliberately different in many cases, e.g. one domain for mail from end users, another for mail sent by a vendor, yet another for another vendor. Why is dmarc different? -- Joseph Brennan Lead, Email and Systems Applications Columbia University Information Technology
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