What is DMARC?
DMARC is a DNS record that tells receiving servers what to do with mail that fails SPF and DKIM alignment for your domain, and where to send reports about it.
Also called: Domain-based Message Authentication, Reporting and Conformance.
The record is a TXT record at _dmarc.yourdomain.com that starts with v=DMARC1 and sets a policy: p=none (monitor only), p=quarantine (treat failures as suspicious) or p=reject (refuse them). A message passes DMARC when SPF or DKIM passes and the domain it passed for matches the visible From: domain.
Google's sender guidelines require bulk senders, those sending more than 5,000 messages a day to Gmail, to publish a DMARC record, and accept a policy of p=none. Gmail and Yahoo have issued permanent rejections to non-compliant mail since November 2025.
A rua= address in the record is worth adding: receivers then send aggregate reports showing every server that sent mail as your domain, which is how you find a forgotten tool that is failing alignment, or someone spoofing you. Start at p=none, read the reports, fix what fails, and tighten the policy once legitimate mail passes.
The SPF, DMARC and MX checker reads the record live and tells you whether it meets the Gmail and Yahoo rules.