How to troubleshoot DKIM failures and which tools to use?

Key findings

• Authentication check: Verifying your DKIM setup is a critical first step. Online tools can check the public key's validity and authoritative DNS for intermittent failures.
• Recipient mailbox testing: Sending emails to major mailbox providers like Gmail and Microsoft (Hotmail/Outlook) is essential. These providers often have different levels of strictness and can reveal specific issues. For instance, Microsoft can be particularly sensitive to DNS or encoding problems. More information on Microsoft DKIM failures is available here.
• Header analysis: Analyzing email headers after sending test emails can pinpoint the exact reason for a DKIM failure. Look for the Authentication-Results header.
• DMARC reports: Utilizing DMARC aggregate reports provides a comprehensive overview of your domain's authentication performance, including DKIM pass/fail rates across various receivers. These reports can show if DKIM success rates are dipping at specific providers.
• Content modification: Forwarding systems or mailing lists that modify message content or headers can invalidate DKIM signatures. This is a common cause of unexpected failures.

Key considerations

• Public key setup: Ensure your DKIM public key is correctly published in your DNS. Any error, even a minor one, can lead to signature validation failures. You can verify your DKIM setup to confirm.
• Body hash mismatch: This is a common failure reason when the email content is altered after signing. Understanding how to fix DKIM body hash failures is essential for many senders.
• DNS propagation: After making changes to your DKIM record, allow sufficient time for DNS changes to propagate across the internet. Delays here can cause intermittent failures.
• Selector usage: Ensure the correct DKIM selector is being used in your email headers, matching the selector specified in your DNS record. Incorrect selectors are a frequent cause of validation issues.
• Tool reliability: While many online DKIM checkers exist, their libraries and adherence to the latest specifications can vary. It's often best to cross-reference results with multiple tools or rely on the authentication results from major mailbox providers directly. The MxToolbox DKIM Check is a widely used resource.

Key opinions

• Using online checkers: Many marketers rely on public online DKIM checkers to quickly confirm their DNS records are published and valid. These tools provide an immediate snapshot of the DKIM setup.
• Testing with major ISPs: Sending test emails to personal Gmail and Outlook/Hotmail accounts is a common practice to see real-world authentication results, as these providers handle a large volume of email and their validation is indicative of broader deliverability.
• Reviewing ESP settings: Marketers often check their ESP's DKIM settings, such as ensuring DKIM is enabled and the provided DNS records are correctly copied. This can sometimes involve generating new keys if previous ones are corrupted or lost.
• DMARC report monitoring: While perhaps more technical, some marketers use DMARC aggregate reports to get a broader view of DKIM authentication rates and identify patterns of failures that might not be obvious from single test sends. Our guide to DMARC reports explains how to use them.

Key considerations

• Impact on deliverability: DKIM failures can severely impact email deliverability, leading to emails being rejected or sent to spam folders, thus reducing campaign effectiveness. This is a crucial aspect of why emails go to spam.
• DNS changes propagation: Marketers need to be aware that DNS changes, especially for DKIM records, can take time to propagate globally, which might cause initial inconsistencies in validation results.
• Third-party modifications: If emails are routed through a system that modifies headers or content after DKIM signing, it can lead to validation failures. This often requires coordination with the modifying service.
• Consistency of tools: Different DKIM checkers may report different results due to varying validation libraries or timing issues. It's often advisable to use a combination of tools and real mailbox tests to get a complete picture. DuoCircle suggests using a DKIM checker or validator to confirm record configuration.

Key opinions

• Hierarchical checking: Experts recommend a structured approach, starting with public key validity checks, then internal authentication tests, followed by sends to various mailbox providers like Gmail and Hotmail.
• Mailbox provider behavior: Gmail is often considered more forgiving, while Microsoft (Hotmail/Outlook) is known to be very sensitive to even subtle DNS issues or encoding problems. This makes Microsoft a good testbed for robust DKIM setup. For more about Gmail SPF/DKIM issues, refer to our guide.
• DNS nuances: Checking authoritative DNS and public DNS for intermittent failures is crucial, as transient DNS issues can cause sporadic DKIM validation problems.
• No single reference implementation: Experts caution that no single mailbox provider perfectly adheres to the DKIM specification, as perfect is the enemy of good. Real-world implementations often prioritize compatibility over strict RFC adherence, meaning some valid DKIM tags might not be universally honored.
• Diagnostic mailboxes: Specialized mailboxes or auto-responders that return detailed authentication results can be highly valuable for systematic debugging. These are often used for troubleshooting DKIM implementation issues.

Key considerations

• DNS issues: Hidden DNS problems can lead to intermittent DKIM failures. Experts recommend deep dives into DNS configurations to uncover subtle issues that affect record lookup and validation.
• Encoding challenges: Certain mailbox providers, particularly Microsoft, can experience DKIM failures due to encoding issues within the email. This can be difficult to diagnose without specific debugging tools or experience.
• Vendor accountability: Determining if a DKIM failure is due to a client's vendor configuration or an internal code issue requires careful isolation testing. This includes ruling out issues with the client's sending infrastructure or their ESP.
• DKIM temperror: Encountering DKIM temperror results can indicate temporary DNS issues or other transient problems. Consistent monitoring is key to distinguishing temporary from persistent failures.

Key findings

• DNS record syntax: The DKIM public key must be published as a TXT record in DNS under a specific format, including the selector. Errors in this syntax are a primary cause of validation failure. Refer to how to fix DKIM record not found errors.
• Canonicalization algorithms: DKIM uses canonicalization (simple or relaxed) for both headers and body to standardize the message before signing and verification. A mismatch in canonicalization or any post-signing modification to the signed parts of the email will result in a body hash mismatch or signature invalidation.
• Key size and type: DKIM keys are typically RSA keys, and their length (e.g., 1024-bit, 2048-bit) can affect compatibility and security. Documentation often specifies recommended key sizes for optimal performance and trust. Tools exist to generate DKIM keys.
• Header fields signed: The h= tag in the DKIM-Signature header specifies which headers are included in the signature. If a signed header is modified after signing, validation will fail.

Key considerations

• RFC compliance: While real-world implementations may vary, adhering to RFCs (like RFC 6376 for DKIM) provides the most robust and widely compatible setup. Deviations can lead to unexpected failures at strict receiving mail servers.
• Aging keys: Regular key rotation is a security best practice. Outdated or compromised keys can lead to authentication issues. Documentation often recommends frequency for key rotation.
• ARC support: For forwarded emails or mailing lists that modify messages, ARC (Authenticated Received Chain) is a protocol designed to preserve authentication results across hops. Implementing ARC can prevent DKIM failures in such scenarios. RFC 8617 details the ARC protocol.
• Error codes: Understanding common DKIM error codes (e.g., permfail, temperror) provided in DMARC reports or email headers is crucial for targeted troubleshooting.