DMARC
A DNS policy that tells receivers what to do with email failing SPF/DKIM checks, and reports who's sending as your domain.
DMARC builds on SPF and DKIM: it sets a policy (none, quarantine, or reject) for messages that fail authentication, and sends you reports on who is sending email as your domain. Start at quarantine and tighten to reject after a few weeks of clean reports.
It's the third leg of the authentication stack required by Gmail and Yahoo for bulk senders. Without it, your legitimate mail is more likely to be spoofed and your deliverability suffers.
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