DMARC, short for Domain-based Message Authentication, Reporting and Conformance, is an email security standard that tells receiving mail servers what to do with messages that claim to come from your domain but fail authentication. It also sends you reports showing who is sending email as you.
How DMARC Works
DMARC builds on two other standards, SPF and DKIM. When a message arrives, the receiving server checks whether it passed SPF or DKIM for the same domain shown in the From line. If neither passes in a way that matches, the message fails DMARC, and the server follows the policy in your DMARC record.
The Three Policies
A policy of none only monitors and asks for reports, without changing how failing mail is handled. A policy of quarantine asks servers to treat failing mail as suspicious, usually sending it to spam. A policy of reject asks servers to refuse failing mail outright. Most domains start at none, review the reports, fix any services that fail, then move to quarantine and finally reject.
Why DMARC Matters
Without DMARC, anyone can send email that appears to come from your domain, which makes phishing in your name easy. Large providers such as Gmail and Yahoo now require a DMARC record from anyone sending email in bulk, and a missing or broken record can hurt delivery for ordinary senders too.
The Domibot DMARC Checker explains your current record and suggests the exact record to use next. The guide DMARC p=none vs Quarantine vs Reject explains when to move from one policy to the next.