How to debug DMARC authentication failure and alignment issues?

Key findings

• Aggregate reports: DMARC aggregate reports are the most critical tool for identifying authentication and alignment failures, providing detailed insights into the specific reasons for DMARC failures and the sources of the emails. They are invaluable for debugging.
• Alignment issues: Even if SPF and DKIM pass individually, DMARC will fail if there is a misalignment between the 'From' header domain and the domains authenticated by SPF or DKIM. This is a common cause of DMARC failures.
• Third-party senders: A frequent source of DMARC failures is emails sent via third-party vendors or IT systems that are not properly configured for SPF or DKIM alignment with your domain. This can often be an overlooked issue.
• Policy enforcement: A DMARC policy set to `p=reject` can mean failed emails are simply discarded. If you're not seeing complaints about missing emails, it might indicate that some mailboxes are not strictly applying your policy (like Gmail sometimes does), or the emails were not critical.

Key considerations

• Analyze reports: Regularly analyze your DMARC aggregate reports to detect any deviations from expected authentication rates. These reports provide invaluable data for diagnosing issues, as highlighted by DuoCircle's guidance on DMARC failures.
• Check all senders: Ensure that every service or vendor sending emails on behalf of your domain is correctly configured for SPF and DKIM, and that these authentication methods align with your `From` domain. For more information on verifying your setup, consider reviewing Suped's guide on how to verify DMARC, DKIM, and SPF setup.
• Understand alignment modes: Be aware of the difference between strict and relaxed alignment modes in DMARC, as this can significantly impact whether an email passes or fails. You can learn more about this in our article on how to concisely explain DMARC passing and identifier alignment.
• Review DNS records: While DNS records might seem stable, a subtle change or a new service could inadvertently introduce DMARC failures. Periodically review your DNS for any unintended modifications.

Key opinions

• Reports are essential: Marketers frequently emphasize that DMARC aggregate reports are the most valuable resource for understanding the specifics of DMARC failures, especially when overall percentages drop.
• Non-bulk sending can fail: Failures aren't limited to large marketing campaigns; even low-volume, non-bulk sending days can experience DMARC authentication issues.
• Vendor accountability: Marketers often find that misaligned emails are sent by IT vendors or other third-party services that have not properly configured their sending for DMARC compliance.
• Alignment is the culprit: When DMARC failures occur, the primary indicator is an alignment issue, regardless of whether SPF or DKIM individually pass authentication.

Key considerations

• Request access to reports: It's crucial to gain access to DMARC reports (XML files) from ISPs like Google to effectively debug failures. Tools that parse these reports are highly beneficial.
• Examine sending streams: Correlate DMARC failure dates with your outgoing mail streams to see if specific sending patterns or sources are linked to the drops in authentication success.
• Verify vendor compliance: Proactively communicate with all third-party email vendors to ensure they understand and meet your DMARC alignment requirements. For more on specific issues, consider reading about how to fix DMARC failures with specific services.
• Policy awareness: Be aware of your DMARC policy (e.g., `p=none`, `p=quarantine`, `p=reject`) and its implications. Even with a `p=reject` policy, some mailboxes might not strictly apply it, affecting your perception of deliverability. Learn more about safely transitioning your DMARC policy.

Key opinions

• Report analysis is key: Experts emphasize that DMARC aggregate reports are the most reliable source of information for understanding authentication failures and diagnosing issues, particularly for recent deviations from 100% success.
• Alignment is the core problem: If DMARC fails, it almost invariably points to an alignment issue with either SPF or DKIM, even if the underlying DNS records appear correct and stable.
• Vendor misconfigurations: A common expectation among experts is that third-party IT vendors or email service providers are often responsible for sending misaligned emails, causing DMARC failures.
• Policy enforcement variations: Experts note that receiving mail servers, such as Gmail, may not always strictly apply a `p=reject` DMARC policy, which can lead to situations where misaligned emails are still delivered.

Key considerations

• Forensic analysis: Dive into the actual DMARC XML files to observe details about the failed emails, including source IP addresses and specific SPF/DKIM alignment checks. This level of detail is necessary for effective troubleshooting, as outlined in our article on understanding and troubleshooting DMARC reports.
• Gradual policy adoption: While a `p=reject` policy is strong, it's safer to start with `p=none` to gather data from DMARC reports before gradually increasing enforcement. Our guide on simple DMARC examples provides more context.
• Identify all sending sources: Thoroughly identify and review all legitimate email sending sources for your domain to ensure they are configured correctly for DMARC. This includes marketing platforms, transactional email services, and internal systems.
• Monitor policy application: Don't assume your DMARC policy is being strictly applied everywhere. Use DMARC reports to confirm how different ISPs are treating your emails, even under a `p=reject` policy.

Key findings

• Alignment requirement: DMARC requires either SPF or DKIM (or both) to pass authentication and for their respective domains to align with the email's 'From' header domain. This alignment is what validates the email's legitimacy.
• Strict vs. relaxed: Documentation specifies that DMARC alignment can be strict (`s`) or relaxed (`r`). Strict requires an exact domain match, while relaxed allows for subdomain matches between the authenticated domain and the 'From' header domain.
• Reporting is crucial: The DMARC specification includes `rua` and `ruf` tags for aggregate and forensic reporting, respectively. These are vital for domain owners to receive feedback on authentication results and identify sending sources causing failures.
• Common pitfalls: DMARC failures are frequently attributed to errors in the DMARC record syntax itself, incorrect SPF and DKIM configurations, and, most commonly, alignment issues.

Key considerations

• Validate records: Always validate your DMARC, SPF, and DKIM records using authoritative tools to ensure they are free of syntax errors and correctly published in DNS. Incorrect syntax is a common cause of failures, as highlighted by 101domain's blog.
• SPF and forwarding: Be aware that email forwarding can often break SPF authentication. For emails that are frequently forwarded, ensuring robust DKIM implementation and alignment becomes even more critical for DMARC pass rates.
• Policy adjustment: To help emails pass DMARC validation, consider using relaxed alignment (`aspf=r`) if strict alignment (`aspf=s`) is causing issues with legitimate sending. As AWS documentation suggests, this can provide necessary flexibility.
• Gradual enforcement: Documentation recommends a gradual enforcement of DMARC policies, starting with `p=none` to collect data, then moving to `p=quarantine` and finally `p=reject` once you have full visibility and confidence in your sending sources.