Definition
DMARC builds on SPF and DKIM to specify what receiving servers should do with email that fails authentication. A DMARC record at _dmarc.yourdomain.com sets the policy: p=none (monitor), p=quarantine (spam folder), or p=reject (block). DMARC also enables aggregate and forensic reports showing who sends email as your domain.
Use the tool: DMARC Checker — check dmarc for any domain.