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3/3. The domain of your ”From: ...@example.com” header fields must have a DMARC policy in the DNS, like:

_dmarc.example.com. IN TXT "v=DMARC1; p=none"

And as a reward for all your hard work to implement this rigmarole, this will do absolutely nothing to reduce anyone's spam!

At best, you can tighten the SPF and DMARC policies to restrict phishing attempts made using your domain, but there are foot-guns…