Technology and tools
What is DMARC?
A DNS policy telling receivers what to do with email that fails SPF and DKIM checks for a domain: nothing, quarantine or reject.
DMARC (Domain-based Message Authentication, Reporting and Conformance) lets a domain owner publish a policy for email that claims to come from their domain but fails SPF or DKIM alignment. The policy can be none (monitor only), quarantine or reject, and the owner receives reports about who is sending as the domain.
Many domains publish DMARC at p=none and never move on, which means spoofed mail is still delivered. Moving to quarantine or reject is what actually stops direct domain spoofing.