Performing Dmarc Policy Enforcement Rollout

DMARC Record Anatomy

v=DMARC1; p=quarantine; pct=25; rua=mailto:[email protected]; ruf=mailto:[email protected]; adkim=r; aspf=r; fo=1

• p: Policy for organizational domain
• sp: Policy for subdomains
• pct: Percentage of messages subject to policy (for gradual rollout)
• rua: Aggregate report destination (daily XML reports)
• ruf: Forensic report destination (per-failure reports)
• adkim: DKIM alignment mode (r=relaxed, s=strict)
• aspf: SPF alignment mode (r=relaxed, s=strict)
• fo: Failure reporting options (0=both fail, 1=either fails)

SPF and DKIM Alignment

• SPF Alignment: The domain in the Return-Path (envelope sender) must match the From header domain
• DKIM Alignment: The d= domain in the DKIM signature must match the From header domain
• Relaxed: Organizational domain match (sub.example.com matches example.com)
• Strict: Exact domain match required

Implementation Steps

Step 1: Inventory All Sending Sources (Week 1-2)

• Audit all systems sending email as your domain (marketing, CRM, ticketing, transactional)
• Document third-party services: Salesforce, Mailchimp, SendGrid, Zendesk, etc.
• Identify internal mail servers, applications, and relay hosts
• Check for shadow IT email sending (departments using unauthorized services)

Step 2: Configure SPF and DKIM (Week 2-4)

• Consolidate SPF record with all legitimate sending IPs and includes
• Ensure SPF record stays under 10 DNS lookup limit
• Generate and publish DKIM keys for each sending source
• Verify DKIM signing works for all outbound mail paths
• Test with MX Toolbox or dmarcian SPF/DKIM validators

Step 3: Deploy DMARC in Monitoring Mode (Week 4-6)

• Publish initial DMARC record: v=DMARC1; p=none; rua=mailto:[email protected]; fo=1
• Wait 1-2 weeks to collect representative aggregate reports
• Analyze reports to identify unauthorized senders and alignment failures
• Fix SPF/DKIM for all legitimate sources showing failures
• Iterate until all legitimate mail passes DMARC

Step 4: Move to Quarantine with pct Tag (Week 6-12)

• Update to quarantine at 10%: v=DMARC1; p=quarantine; pct=10; rua=...
• Monitor for false positives (legitimate mail being quarantined)
• Increase pct gradually: 10% -> 25% -> 50% -> 75% -> 100%
• Each increase: wait 1-2 weeks and review reports before advancing
• Fix any remaining alignment issues discovered at each stage

Step 5: Advance to Reject Policy (Week 12-20)

• After stable quarantine at 100%, move to reject at 10%: v=DMARC1; p=reject; pct=10; rua=...
• Gradually increase pct: 10% -> 25% -> 50% -> 100%
• Monitor closely for legitimate mail being rejected
• Establish emergency rollback procedure (revert to quarantine)
• Apply subdomain policy: sp=reject for subdomains

Step 6: Ongoing Monitoring and Maintenance

• Continuously monitor DMARC aggregate reports
• Add new sending sources before they start sending
• Review forensic reports for spoofing attempts
• Maintain SPF record as sending infrastructure changes
• Rotate DKIM keys annually

Tools & Resources

• EasyDMARC: DMARC monitoring dashboard with aggregate/forensic report analysis
• DMARCLY: SPF, DKIM, DMARC monitoring with auto-DNS updates
• dmarcian: DMARC deployment and management platform
• Valimail: Automated DMARC enforcement with hosted authentication
• MX Toolbox: DNS record lookup and DMARC validator
• Google Admin Toolbox: DMARC check and diagnostic tools

Validation

• DMARC record published and resolving correctly at _dmarc.domain.com
• All legitimate sending sources pass SPF and/or DKIM alignment
• Aggregate reports show >99% legitimate mail passing DMARC
• Spoofed messages from unauthorized senders are rejected
• No legitimate mail blocked after full p=reject enforcement
• Subdomain policy (sp=) also set to reject