What is DMARC?

DMARC, short for Domain-based Message Authentication, Reporting and Conformance, is a published rule that tells other mail servers how to treat messages claiming to come from your domain. It builds on two older checks: SPF, which says which servers may send for the domain, and DKIM, which signs messages. A message passes DMARC when at least one of these checks succeeds and matches the domain in the From line, which is what the reader actually sees. The policy can be none (just report), quarantine (send failures to spam) or reject (refuse them). DMARC also sends you aggregate reports listing every source that sent mail in your name, which is often the first time a company sees all its forgotten senders. On its own it does not stop phishing in general, because attackers simply use a different, similar-looking domain. But it closes the easiest door: exact impersonation of your own address.

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