Glossary / DMARC
What is DMARC?
DMARC tells receiving servers what to do with email that fails SPF and DKIM checks, and sends you reports on who is sending as your domain.
SPF and DKIM detect forged mail. DMARC is the policy layer on top: it states whether failing messages should be delivered anyway, quarantined, or rejected outright.
It also turns on reporting. You receive summaries of who is sending email claiming to be your domain, which is often the first time an organisation discovers the answer.
Start in the monitoring-only mode, read the reports until you recognise every legitimate sender, and only then tighten the policy. Tightening first is how companies accidentally block their own invoices.
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