How can I troubleshoot DMARC failures and identify the cause of authentication issues?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 May 2025
Updated 17 Aug 2025
9 min read
DMARC failures can be incredibly frustrating. One day, your email deliverability seems fine, and the next, you see a sharp drop in your DMARC success rates in tools like Google Postmaster. I've experienced this myself, noting a dip as low as 72% for our domain's DMARC success rate. It immediately makes you wonder what went wrong, especially when your primary email sending platform confirms everything looks correct on their end.
The challenge often lies in identifying all the various platforms that send email on behalf of your domain. Beyond your main email service provider, you might have transactional emails from other services, marketing automation platforms, or even internal systems using your domain for notifications. If any of these sources aren't properly configured with SPF (Sender Policy Framework) or DKIM (DomainKeys Identified Mail), it can lead to DMARC authentication failures. Let's explore how to troubleshoot these issues and get your email authentication back on track.
When you encounter DMARC authentication issues, the first step is to understand what DMARC is looking for. It builds upon SPF and DKIM, requiring that at least one of these mechanisms passes and, crucially, that the domain in the From header (the one users see) aligns with the domain verified by SPF or DKIM. Without proper alignment, even if SPF or DKIM passes, DMARC will fail.
My experience taught me that DMARC is only set up in one place: your domain's DNS. However, every single source that sends email 'from' your domain must have correct SPF and DKIM authentication. This includes email marketing platforms, transactional email services, and even internal applications. For a deeper dive into these protocols, check out a simple guide to DMARC, SPF, and DKIM.
The critical first step is to ensure your DMARC record is correctly published in your DNS. A misconfigured record or one with a typo can immediately lead to widespread authentication failures. You can use a DMARC record generator or checker to confirm your setup.
Common DMARC issues
SPF alignment failure: The Return-Path (or Mail From) domain does not match the From header domain.
DKIM alignment failure: The domain signed by DKIM does not match the From header domain.
Missing or incorrect DNS records: SPF, DKIM, or DMARC records might be absent, incomplete, or contain errors.
Email forwarding: When emails are forwarded, SPF often breaks. DMARC will then rely solely on DKIM.
Leveraging DMARC reports
The most effective way to identify the cause of DMARC failures is to analyze your DMARC reports. These reports, sent to the email address specified in your DMARC record's rua or ruf tags, provide invaluable insights into who is sending email on your behalf and whether those emails are passing or failing DMARC checks. For guidance on how to use them, refer to how to diagnose DMARC failures using DMARC reports.
Initially, these XML reports can seem overwhelming. They contain a lot of data, but the key is to look for the source IP addresses, the domains used in SPF and DKIM, and the DMARC authentication results. This will help you pinpoint which sending sources are failing and why (e.g., SPF failed, DKIM failed, or alignment issues).
When reviewing your DMARC reports, pay close attention to the Authentication-Results header in the raw email source if you're inspecting individual emails. This header provides a summary of SPF, DKIM, and DMARC results for that specific message. For example, a DMARC 'fail' isn't necessarily indicating that your DMARC record is incorrect, but rather that the message itself failed to satisfy your policy, as discussed on Stack Overflow. It helps to understand how to check authentication results in email headers.
Investigating common causes of DMARC failures
A common scenario I've seen is DMARC failures occurring even when SPF and DKIM appear to pass. This almost always points to an alignment issue. DMARC requires that the domain used for SPF (the Return-Path) or DKIM (the d= tag) matches the domain in the visible From header. If these don't align, DMARC will fail the email, regardless of SPF or DKIM passing. This is a frequent cause for why DMARC authentication fails even when SPF and DKIM pass.
Another common pitfall involves third-party sending services. Many organizations use multiple email service providers (ESPs) or other platforms (like marketing automation tools or CRM systems) that send emails on their behalf. Each of these platforms needs to be correctly configured for SPF and DKIM, and critically, they must support DMARC alignment. For instance, if you're using a platform like SendGrid, you need to ensure its settings correctly align with your DMARC policy. You can learn about what causes DMARC bounces and SendGrid authentication issues in our related article.
Misconfigured DNS records are also a frequent culprit. This includes incorrect SPF include mechanisms, expired DKIM keys, or even exceeding the 10-lookup limit for SPF. These issues can lead to authentication failures, making emails susceptible to being flagged as spam or rejected by recipient servers, potentially landing your domain on a blocklist or blacklist. Here's what to review:
SPF considerations
Sender IP addresses: Ensure all legitimate sending IP addresses are included in your SPF record.
DNS lookup limits: Keep your SPF record within the 10 DNS lookup limit to avoid errors such as SPF authentication failed.
A common cause for DMARC failures is when the SPF record in DNS includes too many lookups, leading to a PermError. Microsoft 365, for example, can be sensitive to this. For deeper insight, consider why your emails fail at Microsoft.
DKIM considerations
Valid keys: Ensure your DKIM keys are valid, published correctly, and not expired or revoked, as this causes authentication failures.
Signature integrity: Mismatches in the DKIM body hash (even minor changes to email content) can lead to failures.
Sometimes, emails fail DKIM checks because the email signature doesn't validate, even if the d= (signing domain) is correct. This can be due to modifications in transit or an invalid key. Our article on how to fix DKIM body hash mismatch failures provides more detail.
Advanced troubleshooting with DMARC reports
One of the most valuable resources for troubleshooting DMARC (and other email authentication issues) is your DMARC aggregate reports. These reports, usually in XML format, provide a daily summary of all email traffic originating from your domain. They break down what passed, what failed, and why, including details on SPF and DKIM authentication results and alignment. I can't stress enough how vital these reports are for comprehensive visibility.
While Postmaster Tools provides a good overview, DMARC reports give you granular data, including IP addresses of senders. This allows you to identify unauthorized (spoofed) senders, as well as legitimate senders that simply haven't been configured correctly. If you're managing multiple sending platforms, these reports are your flashlight in the dark, revealing streams of mail you might not even know about. The IETF DMARC Failure Reporting draft offers technical specifics.
In addition to DMARC reports, ensure you have a clear inventory of all your email sending services. Maintaining a spreadsheet or knowledge base of all platforms that send email on behalf of your domain, their configuration URLs, and authentication status can help keep things organized and make future troubleshooting much easier.
Handling specific DMARC failure scenarios
Even with DMARC reports, some issues can be tricky. Sometimes, an internal email sent between domains you control might fail DMARC checks. This can happen if the hostname or mail flow configuration isn't correctly set up to respect DMARC for internal mail. It is important to adjust hostnames or mail server configurations to address this. For Google Workspace users, specific troubleshooting steps are available.
Another scenario is when DMARC reports show failures for legitimate senders, but your SPF and DKIM records appear to be correctly configured. This often indicates a relaxed versus strict alignment issue. DMARC policies can be set to either relaxed (r) or strict (s) mode for SPF and DKIM. In relaxed mode, DMARC passes if the organizational domain matches. In strict mode, the full domain must match. Most often, issues arise with SPF alignment, as noted by 101domain. If you're encountering such an issue, you may want to review why some emails fail DMARC checks despite correct alignment.
Finally, be aware of email forwarding. When an email is forwarded, the Return-Path often changes to the forwarding server's domain, causing SPF to break. In such cases, DMARC relies solely on the DKIM signature to pass. If DKIM also fails or isn't present, the email will likely fail DMARC. This is a common issue for legitimate emails failing DMARC, as discussed by Profund Communications.
Always start with a 'p=none' DMARC policy to gather data without impacting deliverability.
Regularly monitor your DMARC aggregate reports to identify all sending sources.
Maintain a comprehensive list of all third-party email senders for your domain.
Ensure both SPF and DKIM are correctly configured and aligned for every sending service.
Common pitfalls
Ignoring DMARC reports, missing critical insights into email authentication.
Assuming DMARC failures are always malicious; legitimate senders are often misconfigured.
Not accounting for email forwarding, which can break SPF and lead to DMARC failures.
Exceeding SPF DNS lookup limits, causing 'PermError' and authentication issues.
Expert tips
Leverage DMARC forensic reports (RUF) for detailed insights into failed messages, including original message headers.
Utilize Postmaster Tools from major mailbox providers to monitor domain reputation and DMARC success rates.
Consider a DMARC management platform for easier report analysis and proactive alerts.
When troubleshooting, check the DMARC record's placement in your DNS, sometimes it's in the wrong zone.
Expert view
Expert from Email Geeks says implementing a DMARC vendor is ideal for capturing reports and generating dashboards, making troubleshooting much simpler compared to manual analysis.
December 21, 2023 - Email Geeks
Expert view
Expert from Email Geeks says DMARC summary reports are key to identifying which email sources are not properly authenticated.
December 21, 2023 - Email Geeks
Key takeaways for robust DMARC implementation
Troubleshooting DMARC failures requires a methodical approach, starting with understanding the core mechanisms of SPF, DKIM, and DMARC alignment. It involves analyzing DMARC aggregate reports to identify all sending sources and pinpointing authentication and alignment issues. This systematic review allows you to address misconfigurations, whether they stem from your primary ESP or lesser-known third-party senders.
By diligently monitoring your DMARC reports and ensuring all legitimate sending sources are correctly authenticated and aligned, you can significantly improve your DMARC success rates and enhance your domain's email security and deliverability. This proactive stance not only prevents your emails from landing on a blocklist (or blacklist), but also protects your brand reputation from spoofing and phishing attacks.
SendGrid, you need to ensure its settings correctly align with your DMARC policy. You can learn about what causes DMARC bounces and SendGrid authentication issues in our related article.
