Why is DKIM failing at some ISPs but not others, and how can I fix it?

Key findings

• DNS misconfiguration: The primary cause of intermittent DKIM failures is frequently an issue with DNS records, such as incorrect NS (Name Server) entries or the presence of wildcard DNS records that can interfere with proper resolution.
• ISP caching behavior: Some ISPs, like Google, may cache successful DNS responses for longer periods, leading to a perceived consistent pass rate, while other ISPs attempt fresh lookups that expose underlying DNS problems. This explains why DKIM might pass for Gmail but fail for others.
• DKIM public key resolution: DKIM failures can occur if the domain where your public key is published (i.e., the _selector.d=value) is not consistently resolvable across all DNS servers used by various ISPs.
• Not related to shared IPs: Being on a shared IP address generally does not impact DKIM authentication, as DKIM verifies the domain's signing key, not the sending IP.

Key considerations

• Utilize DNS testing tools: Employ tools that provide detailed DNS query results, such as DNSViz, to identify inconsistencies or errors in your DNS records across different authoritative servers. This is crucial for diagnosing intermittent email delivery failures.
• Review NS records and wildcard entries: Work with your IT team or DNS administrator to confirm that all NS records are correctly configured and resolve properly. Investigate any wildcard DNS entries that might be causing unexpected behavior or conflicts.
• Engage your DNS provider: If DNS issues persist, contact your DNS hosting provider (e.g., Azure DNS) with the diagnostic outputs from testing tools. They can investigate why certain DNS servers may not be resolving correctly or if there are delegation problems. Consistent DKIM success rates are key to good deliverability.
• Monitor DMARC reports: DMARC aggregate reports can provide valuable insights into where DKIM failures are occurring and help pinpoint specific ISPs or sending sources experiencing issues, aiding in a more targeted troubleshooting approach.

Key opinions

• Initial confusion: Marketers are often puzzled when DKIM works for some ISPs but not all, initially wondering about factors like return-path resolvability or shared IP addresses.
• Seeking expert assistance: The complexity of DKIM failures often leads marketers to reach out to more technical peers or specialists for help in diagnosing the problem, indicating a gap in their technical knowledge.
• Reliance on testing tools: Many suggest using email testing tools to get a clearer picture of authentication status across different providers, highlighting the importance of diagnostic capabilities for everyday users.
• Perceived problem with specific ISPs: Marketers often frame the issue as specific ISPs (like Yahoo or Hotmail) having problems, rather than an internal misconfiguration, showcasing a need for better understanding of authentication processes.

Key considerations

• Escalating to IT: When facing intermittent DKIM failures, marketers should be prepared to involve their IT department to investigate DNS settings and server configurations, as these are typically beyond marketing's purview.
• Understanding technical feedback: Marketers need to learn how to interpret technical feedback, such as DNS query outputs or DMARC reports, to effectively communicate issues to their technical teams and fix email deliverability problems.
• Leveraging community support: Online forums and communities are valuable resources for marketers to share experiences and receive guidance on complex email authentication issues from peers and experts, as seen on the Spiceworks Community.

Key opinions

• DNS as the root cause: Experts commonly attribute intermittent DKIM failures to issues within the domain's DNS setup, especially when only some ISPs are affected.
• Wildcard DNS record impact: The presence of wildcard DNS records can complicate DNS resolution and lead to ambiguous or incorrect responses, making DKIM validation unreliable.
• Inconsistent authoritative DNS servers: If a domain has multiple authoritative DNS servers, and only some are correctly configured or responsive, this can lead to intermittent failures depending on which server an ISP queries.
• ISP caching differences: Google (and its DNS resolvers like 8.8.8.8) might cache positive DNS responses for longer periods or have more robust resolution mechanisms, which can mask underlying DNS issues that other ISPs quickly uncover.

Key considerations

• Thorough DNS diagnostics: Utilize specialized DNS diagnostic tools to check the health and consistency of your DNS records across all name servers. This can reveal problems such as no DKIM record found errors.
• Correct NS records: Ensure all name server (NS) records for your domain point to existing and correctly configured systems. Typographical errors in NS entries are a common cause of resolution failures.
• Address wildcard entries: If a wildcard DNS record is present and causing issues, consider removing or carefully managing it to avoid conflicts with specific records like your DKIM TXT record, preventing DKIM TempError issues.
• Consult DNS hosting support: If you're using a specific DNS hosting service (e.g., Azure DNS), it's advisable to open a support ticket with them, providing diagnostic outputs to help them investigate and rectify the DNS delegation or record issues, as highlighted by Amazon Web Services documentation on DKIM troubleshooting.

Key findings

• DNS resolution is fundamental: DKIM relies entirely on the recipient server being able to successfully query your domain's DNS for the correct public key record. Any failure in this resolution process will cause DKIM to fail.
• CNAME record requirement: For services like Amazon SES, setting up Easy DKIM requires adding generated CNAME records to your domain's DNS. Failure to correctly add or maintain these records results in DKIM verification issues.
• Signature validation: DKIM authentication fails if the email's signature cannot be found or if the existing signature does not match the one generated using the public key retrieved from DNS.

Key considerations

• Accurate DNS record entry: Manual errors or improper copying/pasting of DNS records (especially TXT records for DKIM) can lead to subtle yet critical misconfigurations that cause validation failures. Always verify the exact syntax as described in DMARC tags.
• DMARC policy impact: When DMARC is in place and configured to require DKIM to pass (as part of alignment), a DKIM failure will lead to a DMARC failure. This highlights the interconnectedness of DMARC, SPF, and DKIM.
• ISP-specific troubleshooting: While general principles apply, sometimes specific ISPs might have unique DNS caching behaviors or resolution paths that require particular attention, making comprehensive troubleshooting across providers essential, as suggested by AWS documentation.