Why is DKIM failing at some ISPs but not others, and how can I fix it?
Michael Ko
Co-founder & CEO, Suped
Published 18 Apr 2025
Updated 19 Aug 2025
11 min read
It can be incredibly frustrating when your DKIM (DomainKeys Identified Mail) authentication works perfectly for some Internet Service Providers (ISPs) like Gmail, but consistently fails for others, such as Yahoo, AOL, or Outlook.com. This inconsistency can lead to email deliverability issues, with your legitimate messages landing in spam folders or being rejected outright. Understanding the root causes of these selective DKIM failures is crucial for maintaining a healthy sending reputation and ensuring your emails reach their intended recipients.
DKIM is a vital email authentication standard that allows the recipient's email server to verify that an email was indeed sent by the domain it claims to be from, and that the message hasn't been tampered with in transit. It involves a cryptographic signature added to the email header, which the receiving server checks against a public key published in your domain's DNS (Domain Name System) records. When this verification fails, it signals to the receiving ISP that something might be wrong with the email, leading to deliverability problems.
I've seen many cases where businesses struggle with this exact scenario, often finding that their DKIM passes flawlessly with Gmail, which is generally very forgiving, but hits a wall with others. This usually points to underlying DNS issues or subtle differences in how ISPs interpret and cache DNS information. Let's dive into the specific reasons why these failures occur and what steps you can take to diagnose and resolve them.
Understanding these nuances is key to ensuring your email campaigns and transactional messages consistently land in the inbox, no matter the recipient's provider. If you're encountering selective DKIM failures, it's a sign that your DNS configuration might need a closer look, especially how different DNS servers around the world are resolving your records.
One of the most frequent culprits behind DKIM failing at some ISPs but not others is a misconfigured or inconsistently propagated DNS record. While DKIM itself is about cryptographic signatures, the public key required for verification lives in your DNS. If that record isn't accessible or is incorrect for certain DNS resolvers, the DKIM check will fail.
Sometimes, a domain might have multiple authoritative DNS servers. If only some of these servers are configured correctly with the DKIM record, or if there's a typo in some of the Name Server (NS) entries at your domain registrar, certain ISPs may query a DNS server that doesn't have the correct information. Google's DNS (8.8.8.8) might be more resilient and able to find the correct record even with minor issues, or it might cache successful lookups for longer, giving the impression that everything is fine when it's not. Other ISPs, particularly those with less aggressive caching or different DNS resolution paths, might hit the misconfigured server and fail the DKIM check.
Another common issue is the presence of wildcard DNS records. While convenient, a wildcard entry can sometimes interfere with specific DNS lookups, especially for DKIM records, leading to ambiguous or incorrect responses for some resolvers. This can make the output of DNS queries appear "messy" or unresolved (NXDOMAIN) for specific DKIM selectors, even if the record theoretically exists. This can be particularly true if your DNS is hosted with providers like Azure DNS, where configurations might be more complex.
When encountering these issues, checking your DNS records with various tools and from different locations is essential. This helps you identify if the problem is with your DNS configuration, the authoritative servers, or simply how different resolvers are caching or interpreting your records. The output from tools like DNSViz or Learn DMARC can provide valuable insights into any inconsistencies or errors in your DNS delegation.
Message alterations and other factors
While DNS issues are a primary cause, other factors can lead to intermittent DKIM failures. One such factor is alterations to the email body or headers after the DKIM signature has been applied. DKIM works by hashing parts of the email (including certain headers and the message body) and then signing that hash with a private key. If the email content changes even slightly after signing, the recipient's server will calculate a different hash, causing the DKIM verification to fail. This often happens with mailing lists or email forwarding services that modify messages by adding footers, disclaimers, or tracking pixels.
High-ASCII characters or encoding issues can also contribute to DKIM body hash failures. Different email clients or receiving servers might interpret or re-encode these characters differently, leading to a mismatch in the calculated hash. For instance, an email containing special characters might be hashed one way by the sending server and another way by a particular receiving ISP, causing the DKIM signature to appear invalid. This is why it's important to use standard encoding and test your emails across various environments. We have a dedicated guide on how to fix DKIM body hash failures.
Furthermore, a lack of DMARC alignment can play a role. While DKIM itself might pass, DMARC requires that the domain in the From: header (RFFC 5322.From) align with the domain used in the DKIM signature (the d= tag). If these domains do not align, DMARC can fail, even if the underlying DKIM signature is technically valid. Some ISPs might be stricter about DMARC alignment than others, leading to differential deliverability outcomes. This is often the case with providers like Yahoo and AOL, which have stronger DMARC enforcement.
Lastly, being on a shared IP address generally should not cause selective DKIM failures, as DKIM is domain-based, not IP-based. However, if the shared IP has a poor reputation or is blocklisted (or blacklisted), it could influence deliverability regardless of DKIM status. Nonetheless, an intermittent DKIM failure typically points to a DNS or message alteration issue rather than the IP itself.
Troubleshooting steps
Diagnosing these intermittent DKIM failures requires a methodical approach. Start by using an email deliverability tester or online tools to send a test email to various recipient domains, especially those where you've observed failures (like Outlook.com or Yahoo Mail). Analyze the full email headers for each test. Look for authentication results (DKIM, SPF, DMARC) and specific error messages.
If the issue is DNS related, you'll need to scrutinize your DKIM DNS record. Ensure the TXT record for your DKIM selector is correctly published and that all authoritative DNS servers for your domain are serving the identical, correct record. You can use tools that perform DNS lookups from various geographical locations to check for inconsistencies. If you find discrepancies, your DNS hosting provider or IT team will need to correct the records and ensure proper propagation.
For message alteration issues, you'll need to identify any intermediaries that might be modifying your emails after they leave your sending server. This could be an ESP (Email Service Provider), a CRM, or even an internal system. Many ESPs offer options to prevent such modifications for DKIM-signed messages. If you're using a mailing list, consider implementing ARF (Authentication-Results-Forwarding) or investigating why the list is breaking your DKIM signature. In some cases, adjusting your DKIM canonicalization settings (relaxed/simple) might help, though simple canonicalization is generally recommended for strict adherence.
Finally, ensure your DMARC policy is correctly configured and that your DKIM record aligns with your From: header domain. If you have a strict DMARC policy (p=quarantine or p=reject), any DKIM failure, even a partial one, could lead to emails being rejected. DMARC reports can be invaluable here, providing data on authentication results from receiving ISPs.
Prevention and best practices
Preventing selective DKIM failures often boils down to diligent setup and continuous monitoring. Regular checks of your DNS records and email authentication status are crucial. Use a DMARC monitoring service to get aggregate and forensic reports that show you exactly which emails are failing and why, across different ISPs. These reports can pinpoint authentication issues, including DKIM failures, and help you understand the specific reasons for non-compliance.
Ensure your DKIM record is always published correctly and that there are no conflicts, such as wildcard entries, that could create resolution issues. If you use multiple DNS servers (e.g., primary and secondary), double-check that the DKIM record is propagated consistently across all of them. DNS propagation can take time, so patience is key after making changes, but using tools to verify instant propagation can save headaches.
Work closely with your email service provider to understand their DKIM implementation and ensure your sending domain is properly configured on their end. They often provide specific instructions for setting up DKIM, which can vary. If you're sending from a subdomain, ensure your DKIM record is correctly set up for it, as this can sometimes be overlooked. Our guide DKIM for a subdomain may be helpful.
Lastly, be mindful of any automated processes that might alter your emails. This includes email marketing platforms that add tracking links or footers, or even security solutions that might modify content. Ensure that these processes are DKIM-aware and don't inadvertently invalidate your signatures. Proactive monitoring and adherence to best practices will significantly reduce intermittent DKIM failures and improve your overall email deliverability.
Conclusion
Resolving selective DKIM failures often comes down to meticulous DNS configuration and understanding how different mail servers interact with your records. While it can seem complex, especially when only some ISPs are affected, the solution usually lies in correcting DNS inconsistencies or ensuring your emails aren't modified post-signing. By systematically checking your DNS, analyzing email headers, and implementing DMARC monitoring, you can identify and fix these issues, leading to more consistent and reliable email deliverability across all providers.
Remember, email authentication protocols like DKIM, SPF, and DMARC are designed to build trust between sending and receiving mail servers. When one component is misconfigured or inconsistent, it erodes that trust, leading to messages being flagged as suspicious. A consistent and valid DKIM signature is a cornerstone of good email deliverability, ensuring your emails are recognized as legitimate by all major ISPs.
Views from the trenches
Best practices
Ensure all authoritative DNS servers for your domain host the identical DKIM TXT record, to prevent inconsistencies across resolvers.
Regularly monitor your DMARC reports to catch any intermittent DKIM failures reported by various ISPs and identify patterns.
Use email testing tools to send messages to diverse mailbox providers, examining full headers for DKIM authentication results.
Communicate with your email service provider about their specific DKIM implementation details and any potential content modification policies.
Common pitfalls
Overlooking subtle DNS configuration errors, such as typos in NS records or inconsistent propagation across DNS servers, which some ISPs might detect.
Not accounting for email content modifications by forwarding services, mailing lists, or marketing platforms that invalidate DKIM signatures.
Ignoring the impact of wildcard DNS entries, which can sometimes interfere with specific DKIM record lookups.
Assuming that DKIM passing on one major ISP (like Gmail) means it's universally correct, neglecting stricter validation by other providers.
Expert tips
If your DKIM fails intermittently, it's very often a DNS problem. Check all your name servers.
Wildcard DNS records can cause issues with DKIM. Try removing them if possible to see if it resolves the problem.
Google's DNS might cache good responses for longer, which can hide underlying DNS configuration problems from your perspective.
If NS records are pointing to non-existent systems, it will cause intermittent DKIM failures, as some ISPs will fail to resolve.
Marketer view
Marketer from Email Geeks says they were seeing DKIM occasionally failing at Yahoo, AOL, Comcast, and Hotmail, while always passing at Gmail, and wondered if shared IPs affected it or if the Return-Path/sending domain being unresolvable was the issue.
2022-01-19 - Email Geeks
Expert view
Expert from Email Geeks says that DKIM failures are common, but are unlikely due to an unresolvable Return-Path or sending domain. They suggest that high-ASCII characters in the message body being hashed differently by various receivers is a more typical cause of intermittent failures, but the main issue is likely an inconsistent DNS configuration.
Gmail, but consistently fails for others, such as 
Yahoo, AOL, or 
Outlook.com. This inconsistency can lead to email deliverability issues, with your legitimate messages landing in spam folders or being rejected outright. Understanding the root causes of these selective DKIM failures is crucial for maintaining a healthy sending reputation and ensuring your emails reach their intended recipients.
Azure DNS, where configurations might be more complex.
