Summary
Even with correctly configured SPF and DKIM records, some emails can still fail DMARC checks. This often stems from subtle alignment issues, third-party sending practices, or intermittent DNS problems rather than outright misconfigurations. While a small percentage of failures can be normal, a higher rate warrants investigation to ensure optimal email deliverability and sender reputation.
Key findings
- Alignment issues: DMARC requires either SPF or DKIM to align with the From: domain. If SPF alignment fails (e.g., Return-Path is from a third-party sender like amazonses.com) and DKIM also fails to align, the email will fail DMARC, even if SPF and DKIM authentication technically passed.
- Third-party senders: Email Service Providers (ESPs) or other third-party services sometimes use their own domains for the Mail From (Return-Path) address or sign emails with their own DKIM keys, leading to DMARC alignment failures. This is a common challenge with services like Amazon SES or Mimecast, as noted in various discussions regarding DMARC validation for SPF or DKIM.
- Intermittent DNS problems: Temporary DNS resolution issues, such as packet loss (common with UDP for DNS queries), can cause intermittent SPF or DKIM validation failures, leading to DMARC failures for a small percentage of emails.
- Configuration out of sync: Occasionally, certain sending systems or servers within a larger infrastructure may not have the correct SPF or DKIM configurations applied, leading to a small number of emails failing DMARC checks, as if using default settings.
Key considerations
- DMARC reporting analysis: Leverage DMARC aggregate and forensic reports to identify patterns in failures, such as specific sending IPs, return paths, or third-party services causing the misalignment. Tools for understanding and troubleshooting DMARC reports are crucial for this.
- Custom MAIL FROM domain: When using services like Amazon SES, configure a custom MAIL FROM domain to ensure SPF alignment. This involves setting up a CNAME record that points to the SES subdomain, so the Return-Path aligns with your sending domain.
- DKIM configuration review: Ensure your DKIM setup explicitly uses your domain for signing, especially with third-party senders. Verify that your DKIM records are correctly configured and that the domain in the DKIM signature (d=) matches your From: domain. Further details on fixing common DMARC failures are available in this article on DMARC failures.
- DNS health check: Regularly check your DNS health, ensuring all name servers are in sync and DNS resolvers are responding quickly. Slow or inconsistent DNS responses can lead to sporadic authentication failures.
What email marketers say
Email marketers often encounter DMARC failures despite seemingly correct SPF and DKIM setups, particularly when dealing with high-volume sending through ESPs like Amazon SES. Their primary concern is often the impact on deliverability and how to minimize these seemingly random failures without compromising legitimate email flow.
Key opinions
- Small failure rates are normal: A very small percentage of DMARC failures, like 0.0005%, is often considered an acceptable anomaly due to the inherent nature of internet protocols and occasional network glitches.
- Third-party branding: Marketers frequently find that ESPs like SES or Mimecast might automatically use their own domains for the Return-Path or DKIM signatures, which can cause DMARC misalignment, especially for automated system emails or bounces.
- Lack of detailed bounce information: A common frustration is the lack of specific error messages from providers like Amazon SES, which often provide generic bounce notifications without enough detail to troubleshoot DMARC failures effectively.
- DNS weirdness: Intermittent DMARC failures, especially when both SPF and DKIM fail for a tiny fraction of emails, often point towards transient DNS resolution issues rather than persistent configuration errors.
Key considerations
- Analyze DMARC reports: Despite low percentages, marketers should closely examine DMARC aggregate reports to pinpoint the source IPs and specific failure types. This helps diagnose whether it's an internal issue or related to an external service.
- Configure custom sending domains: When using ESPs, ensure proper setup of custom MAIL FROM domains and DKIM CNAMEs. This is critical for achieving DMARC alignment and preventing your emails from going to spam due to SPF and DKIM alignment failures.
- Consult ESP support: If using a paid ESP, leverage their support channels. They often have insights into their platform's specific behaviors, such as how their systems handle bounces or automated emails that might impact DMARC.
- Monitor DNS health: Regularly check your DNS infrastructure for stability and speed. Inconsistent DNS resolution can lead to what appears to be random DMARC failures and affect overall deliverability.
Marketer view
Email marketer from Email Geeks observes that with Amazon SES, even when SPF and DKIM are correctly aligned and pass for the vast majority of emails, a tiny fraction still fail DMARC checks. This happens because the Mail From (Return-Path) and DKIM signatures for these specific emails revert to amazonses.com, causing alignment issues. This behavior mirrors issues seen with other third-party senders, such as Mimecast, which might automatically sign postmaster@domain.com emails with their own authentication.
Marketer view
Email marketer from Email Geeks suggests checking for consistency in reporting data. It's possible that a small portion of the sending infrastructure might be out of sync with configurations, leading to a default setting being used for a tiny percentage of emails. This could explain the sporadic DMARC failures despite overall correct setup.
What the experts say
Experts in email deliverability acknowledge that a perfect DMARC pass rate is rarely achievable. They highlight that a minimal percentage of failures is an inherent part of the DMARC protocol and how DNS operates. The focus for experts shifts from eliminating every failure to understanding their cause and ensuring they don't significantly impact deliverability or indicate a security vulnerability.
Key opinions
- Inherent protocol behavior: A small percentage of DMARC failures is expected and an intrinsic part of how the DMARC protocol works. It's not always indicative of an underlying configuration problem.
- DNS unreliability: DNS queries, being UDP-based, can occasionally experience packet loss. This means that a DNS response might fail to reach the recipient's server, leading to an intermittent SPF or DKIM failure and subsequently, a DMARC failure.
- Not always a configuration problem: When failures are isolated to a single email out of thousands from the same IP, and SPF is also failing, it strongly suggests a transient network or DNS issue rather than a consistent misconfiguration on the sender's end.
- DNS rate limiting: Some DNS providers might implement rate limiting, which could sporadically prevent successful DNS lookups for SPF or DKIM records during high-volume sending, contributing to intermittent failures.
Key considerations
- Verify DNS synchronization: Ensure all your name servers are fully in sync and that there are no broken DNS entries. This consistency is vital for reliable SPF and DKIM lookups.
- Monitor DNS resolver response times: Keep an eye on the responsiveness of your DNS resolvers. Slow responses can lead to timeouts and authentication failures. For more on this, consider our guide on hidden SPF DNS timeouts.
- Consult DNS provider about rate limiting: If you're not using a managed DNS service like Route53, directly inquire with your DNS provider about any rate limiting policies that might affect high-volume DNS queries for email authentication.
- Understand DMARC's tolerance for failure: Recognize that a 100% DMARC success rate is often an unrealistic goal. Focus on keeping the failure rate negligibly low and understanding the types of failures that occur. Further troubleshooting for general DMARC issues can be found in our guide on DMARC failure troubleshooting.
Expert view
Expert from Email Geeks notes that it is interesting when both SPF and DKIM fail simultaneously, as it suggests a problem that is preventing proper DNS resolution. This dual failure points away from a specific authentication record issue and more towards a fundamental network or DNS problem preventing the records from being looked up.
Expert view
Expert from Email Geeks states that a small percentage of mails failing DMARC, even with everything correctly configured, is not unexpected. They emphasize that this is part and parcel of how DMARC works and that some legitimate mail will inherently fail and be rejected due to various transient factors.
What the documentation says
Official documentation and technical specifications for DMARC, SPF, and DKIM provide the foundational understanding for these authentication protocols. They detail the precise mechanisms of alignment and the conditions under which an email is deemed DMARC compliant or non-compliant, highlighting potential areas where even correctly authenticated emails might fail due to alignment mismatches or transient errors.
Key findings
- SPF alignment requirement: For DMARC, SPF alignment requires that the RFC5321.MailFrom domain (also known as the Return-Path or Envelope-From) matches the RFC5322.From domain (the visible From address) or a subdomain thereof. If the Mail From domain is from a third-party ESP and doesn't align, SPF will fail DMARC validation.
- DKIM alignment requirement: DKIM alignment for DMARC means the 'd=' tag in the DKIM signature must match the RFC5322.From domain or a subdomain. If a third party signs with their domain, DKIM will authenticate but fail DMARC alignment.
- DMARC policy evaluation: DMARC only requires either SPF or DKIM to pass alignment. If both authentication methods pass but fail alignment, the DMARC check will fail, and the policy (e.g., reject or quarantine) will be applied. This is a common cause for DMARC authentication failing even when SPF and DKIM pass.
- Role of DMARC reports: DMARC reports (RUA and RUF) are designed to provide insights into email authentication results, including failures, allowing domain owners to diagnose and correct issues. They specify whether SPF and DKIM passed authentication, failed, or passed but failed alignment.
Key considerations
- Implement custom MAIL FROM: To achieve SPF alignment with third-party senders like Amazon SES, it is necessary to set up a custom MAIL FROM domain. This typically involves configuring a CNAME record that allows your domain's Return-Path to point to the ESP's domain while appearing to originate from your domain for SPF checks. This is detailed in AWS documentation on DMARC and SES.
- Ensure DKIM domain consistency: Verify that your DKIM configuration ensures that the signing domain (d= tag) in the DKIM header always matches your primary sending domain, not the ESP's default domain. This is crucial for DKIM alignment.
- Monitor DMARC reports for anomalies: Even with seemingly perfect configurations, DMARC reports can reveal a small percentage of failures due to transient DNS issues or system quirks. Regularly analyzing these reports helps maintain optimal deliverability and domain reputation, as discussed in our guide to improving domain reputation.
- Understanding aggregate data: DMARC aggregate reports (RUA) provide statistical overviews, which can highlight recurring patterns of failure, even for small volumes. Understanding how to interpret these XML reports is key to diagnosing obscure issues.
Technical article
RFC 7489 (DMARC) specifies that DMARC authentication passes if either SPF or DKIM passes *and* is in alignment with the RFC5322.From header field. It states that if both authentication mechanisms pass but fail alignment, the DMARC check results in a fail, triggering the defined policy.
Technical article
The Internet Engineering Task Force (IETF) documentation on SPF (RFC 7208) highlights that SPF checks validate the RFC5321.MailFrom domain. For DMARC alignment, this domain must match the RFC5322.From domain, which is a common point of failure when third-party senders are involved and use their own Return-Path.
Related resources
14 resources
Related pages
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