After seven years of observing organizations struggle with Domain-based Message Authentication, Reporting, and Conformance (DMARC) implementations, a clear pattern has emerged. The most common reason for these failures? Difficulty identifying legitimate email sources for authentication.
The DMARC Challenge: Identifying Legitimate Email Sources
DMARC reports are invaluable for understanding how email authentication is performing. However, they are often cluttered with irrelevant data:
- Emails relayed by recipients who forward messages to other mailboxes
- Spam sent by malicious actors
This noise makes it challenging for organizations to focus on the critical task of identifying legitimate email sources and authenticating them using SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). Missteps in this phase lead to stalled DMARC projects and incomplete protection against phishing and spoofing attacks.
If you don’t have a DMARC expert guiding your implementation, don’t worry. Here’s a step-by-step guide to help you succeed.
Step 1: Identify Obvious Email Sources
Start by analyzing your DMARC reports for recognizable email sources. Common ones include:
- Corporate email platforms (e.g., Microsoft 365, Google Workspace)
- Marketing platforms (e.g., Mailchimp, Salesforce)
- Customer support systems (e.g., Zendesk)
Ensure these sources are properly authenticated.
Troubleshooting Common Authentication Errors:
- SPF Issues:
- Check if the sending IP address is included in your SPF record.
- Verify whether the SPF record is configured for the correct domain. Subdomains may need separate SPF entries.
- DKIM Issues:
- Confirm the DKIM key is published correctly in your DNS.
- Watch for conflicts where multiple systems use the same DKIM selector but different private keys.
By systematically resolving these errors, you can ensure legitimate sources are authenticated effectively.
Step 2: Handle Unknown Sources with DKIM Signatures
Unknown sources in your DMARC reports may still have a DKIM signature using your domain. In most cases, these emails are forwarded by recipients’ email systems.
Why This Matters:
Forwarding can disrupt SPF authentication but often retains DKIM signatures. Because forwarded emails typically have some level of whitelisting at the recipient's destination, you usually don’t need to take further action for these sources.
Step 3: Investigate Unknown Sources Without DKIM Signatures
When an unknown source lacks your domain’s DKIM signature, further investigation is required:
- Examine the Envelope-To Field and Look Up the Organization Sending the DMARC Report:
Check the destination domain in the DMARC report.
- If the emails are addressed to your domain, inspect their content using your anti-spam systems.
- If the e-mails are not addressed to your domain, Search online or check internal ticketing systems to determine if the sender is a trusted supplier or partner. If necessary, contact the sender for clarification.
These steps help differentiate legitimate but unrecognized sources from malicious activity.
When to Move to a Quarantine or Reject Policy
Once your DMARC pass rate reaches 99% for legitimate sources, you’re ready to enforce stricter policies. Transition to a p=quarantine or p=reject policy to block unauthorized emails, significantly reducing your domain’s vulnerability to spoofing attacks.
The Key Takeaway
The success of a DMARC implementation hinges on your ability to identify legitimate email sources and authenticate them. Noise in DMARC reports can make this task daunting, but by following the steps outlined above, you can simplify the process and achieve a secure email environment.
Remember: DMARC is not just a technical protocol; it’s a journey toward email authentication excellence. With persistence and the right approach, you can protect your domain from abuse and ensure your DMARC project is a success.