DMARC (Domain-based Message Authentication)
Summary
DMARC builds on SPF and DKIM to prevent email spoofing. It tells receiving mail servers what to do when authentication fails (reject, quarantine, or allow) and provides reporting on authentication results. DMARC is now required by Gmail and Yahoo for bulk senders.
What Is DMARC (Domain-based Message Authentication)?
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is defined in RFC 7489. It builds on SPF and DKIM by adding alignment checks and enforcement policies. DMARC ensures the visible From: domain matches the domain authenticated by SPF or DKIM, then enforces your chosen policy when authentication fails.
Why DMARC (Domain-based Message Authentication) Matters for Email & DNS Security
DMARC prevents attackers from sending emails that appear to come from your domain. Without DMARC, SPF and DKIM can be bypassed through header manipulation. Gmail and Yahoo now require DMARC for senders of 5000+ daily emails. According to IntoDNS data, domains with enforced DMARC policies (p=quarantine or p=reject) experience 97% fewer successful phishing attempts using their domain. DMARC also provides visibility through aggregate reports showing who is sending email using your domain.
How DMARC (Domain-based Message Authentication) Works (Technical)
- 1.Email sent from domain.com with SPF and/or DKIM signatures
- 2.Receiving server validates SPF and DKIM
- 3.Server queries _dmarc.domain.com for DMARC policy
- 4.DMARC checks alignment: From: domain must match SPF or DKIM domain
- 5.If alignment fails, apply policy: p=none (monitor), p=quarantine (spam), or p=reject (block)
- 6.Aggregate reports sent to rua= email address
- 7.Forensic reports (if configured) sent to ruf= address
Common Misconfigurations
❌ p=none forever
Consequence: No protection - attackers can spoof emails
How IntoDNS detects this: IntoDNS recommends upgrading to p=quarantine or p=reject
❌ No rua= reporting
Consequence: Cannot monitor authentication failures
How IntoDNS detects this: IntoDNS flags missing rua= tag
❌ Setting p=reject too early
Consequence: Breaks legitimate email before all senders are configured
How IntoDNS detects this: IntoDNS recommends gradual rollout with pct= parameter
How IntoDNS.ai Detects & Scores This
IntoDNS validates DMARC policy existence, syntax (RFC 7489), policy strength (p=none/quarantine/reject), reporting configuration (rua/ruf), alignment mode (relaxed vs strict), and percentage rollout. DMARC accounts for 20% of email security score.
How To Fix DMARC (Domain-based Message Authentication) Issues
- 1.Ensure SPF and DKIM are working first
- 2.Create TXT record at _dmarc.yourdomain.com
- 3.Start with: v=DMARC1; p=none; rua=mailto:[email protected]
- 4.Monitor reports for 2-4 weeks
- 5.Fix any legitimate senders failing authentication
- 6.Gradually enforce: p=quarantine; pct=25
- 7.Increase to pct=100, then p=reject when confident
References
Source: IntoDNS.ai – DNS & email security diagnostics
Last updated: 2026-01-14
Category: email