DMARC

Domain-based Message Authentication, Reporting & Conformance

Policy record sitting on top of SPF + DKIM. Tells receivers what to do when both fail (none / quarantine / reject) and where to send aggregate or forensic reports.

_dmarc.acme.dev. IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@acme.dev"