Definition
DMARC
Domain-based Message Authentication, Reporting & Conformance
DMARC is a DNS record published at _dmarc.your-domain.com that tells receiving mail servers what to do with messages claiming to come from your domain that pass neither SPF nor DKIM, and that has those servers send you a daily report of what they received in your name.
DMARC replaces neither SPF nor DKIM; it is built on top of them. A message passes DMARC if it passes either check and the domain that passed it matches the domain your recipient sees in the From field. That second condition, alignment, is what does the work: without it a fraudster would simply sign with their own domain and pass.
The record carries a policy (p=none, p=quarantine or p=reject) and a rua address that aggregate reports are sent to. Publishing a record at p=none protects nothing: it is an alarm fitted but never wired in.
_dmarc.example.com. IN TXT
"v=DMARC1; p=reject; rua=mailto:reports@example.com; adkim=s; aspf=s"- Also known as
- DMARC, DMARC record, _dmarc, Domain-based Message Authentication, Reporting and Conformance
- Standard
- RFC 7489