What causes DKIM errors during double DKIM implementation and how can they be fixed?
Matthew Whittaker
Co-founder & CTO, Suped
Published 26 Jul 2025
Updated 18 Aug 2025
8 min read
Implementing DomainKeys Identified Mail (DKIM) is crucial for email authentication, helping to verify sender identity and prevent spoofing. When you delve into more complex setups, such as "double DKIM," where multiple services sign your emails, you might encounter unexpected errors. These issues can be particularly perplexing, especially when your initial DKIM setup seemed flawless for most recipients but fails for a specific few, leading to a DKIM = Unknown status in email headers.
Double DKIM implementation, often necessary when using a third-party email service provider (ESP) in conjunction with your own mail transfer agent (MTA) or another sending platform, means your emails carry two valid DKIM signatures. While this adds layers of verification, it also introduces potential points of failure that require careful attention and troubleshooting to maintain strong email deliverability.
Double DKIM signing occurs when an email is authenticated with two separate DKIM signatures from different domains or selectors. This setup is becoming increasingly common, especially with the stricter email authentication requirements enforced by major mailbox providers like Gmail and Yahoo. It typically involves your primary domain's DKIM signature alongside a signature from a sending service, ensuring that both the originating domain and the sending platform verify the email's integrity.
The primary purpose of implementing double DKIM is to enhance email deliverability and security. By having multiple signatures, you provide additional proof of authenticity, which can help your emails bypass spam filters and improve inbox placement. This is particularly relevant when you're sending emails through a third party, where the third party adds their own DKIM signature to your messages. This dual authentication strengthens your email's trustworthiness.
While beneficial, setting up double DKIM signing requires precise configuration to avoid errors. Each DKIM record, whether for your domain or a third-party service, must be correctly published in your DNS. Any misconfiguration can lead to validation failures, causing your emails to be marked as spam or rejected, thus undermining the very purpose of implementing DKIM.
Causes of DKIM errors in double DKIM setups
When implementing double DKIM, several factors can lead to authentication failures. Understanding these common causes is the first step toward effective troubleshooting and ensuring your emails reach their intended recipients. These issues often stem from how the DKIM records interact or how emails are handled in transit.
DNS misconfigurations: A common cause of DKIM failures is incorrect DNS configuration. This can include typos in the TXT record, using the wrong selector, or having missing records. Some DNS providers automatically append the domain to record names, which can lead to duplication errors if not handled carefully.
Key length limitations: Certain DNS providers have limits on the length of TXT records. If your DKIM key, especially a 2048-bit key, exceeds this limit, it may be truncated or improperly stored, causing validation failures. This often results in an "invalid RSA public key error" or DKIM temperror.
Body hash modification: DKIM generates a hash of the email body. If the email content is altered even slightly after signing but before reaching the recipient, the hash will no longer match, leading to a DKIM body hash failure. This can occur due to automatic forwarding, email list management systems, or even some MTAs adding footers or making minor encoding changes. This is a common reason why DKIM appears broken for some users but not others.
DKIM alignment issues: For DKIM to pass DMARC, the domain in the DKIM signature must align with the domain in the "From" header. If you are using a third-party sender that signs with its own domain, and it doesn't match your "From" domain, it can lead to DKIM alignment failures, even if the DKIM signature itself is valid. This is often seen when Klaviyo DKIM fails.
Double DKIM setups specifically can suffer from these issues because there are two signatures that need to be verified independently. If one signature fails, or if the interaction between the two causes an issue, it can lead to problems, particularly with more stringent receivers like Yahoo Mail.
Common DKIM error messages
DKIM = Unknown: Often indicates a problem retrieving or validating the DKIM public key, or the email was modified in transit.
DKIM = Neutral: The signature exists but couldn't be verified, possibly due to a minor issue or policy setting. This is less severe but still requires attention.
DKIM = Fail: The signature was present but failed validation, often pointing to a mismatched body hash, an invalid key, or a severe DNS error.
Sometimes, these errors only manifest for a single recipient or a particular mail server, suggesting a caching issue on the recipient's end, a specific configuration on their server, or intermittent DNS propagation delays affecting only certain resolvers.
Troubleshooting double DKIM failures
Diagnosing DKIM failures requires a systematic approach, especially in a double DKIM setup. The goal is to pinpoint exactly where the authentication chain is breaking. Start by obtaining the full email headers of a failed message, as these contain crucial clues about the verification process at the recipient's server.
Next, use a DNS lookup tool to verify the published DKIM records for both your primary domain and any third-party services. Ensure that the selectors used in the email headers match the selectors in your DNS. Check for any extra characters, missing dots, or incorrect record types. A single typo can invalidate your DKIM signature, even if it looks correct to the naked eye.
When trying to identify the cause of DKIM failure, consider using Google's tools to test emails. Google's systems are known for being robust and providing clear authentication results. If an email passes DKIM with Gmail but fails elsewhere, it might indicate an issue specific to the recipient's mail server or a localized caching problem. Remember that if you only see an error for a single recipient, it's worth checking if it's a transient issue or specific to their setup.
Scenario: DKIM failure
Problem: Email shows "DKIM=Unknown" or "DKIM=Fail" in headers for some recipients, particularly with a double DKIM setup.
Observation: One DKIM signature passes, but the second one, or the overall authentication, reports a failure or an unknown status.
Diagnostic approach
Check email headers: Analyze the "Authentication-Results" section. Look for specific error messages for each DKIM signature. Pay attention to the reported domain and selector.
Verify DNS records: Use a DNS lookup tool to confirm that both DKIM TXT records are correctly published and accessible. Check for any missing records or syntax errors.
Content integrity: Consider if the email content or headers are being modified post-signing, leading to a body hash mismatch. This can happen with certain ESPs or MTAs.
Effective solutions for DKIM problems
Once you've diagnosed the root cause of your double DKIM errors, applying the correct fixes is crucial. The solutions typically involve meticulous configuration adjustments and ongoing monitoring to ensure consistent email authentication.
If DNS misconfigurations are the culprit, carefully re-enter the DKIM TXT records, paying close attention to selectors and key values. For key length issues, you might need to consult your DNS provider's documentation or consider using a shorter key if feasible, though 2048-bit keys are preferred. If DKIM body hash mismatch is occurring, investigate any systems that might modify your email content between sending and reception. This often requires adjusting settings within your ESP or MTA to prevent unintended alterations, or understanding how different MTAs handle content modifications, such as Microsoft Office 365 DKIM signatures.
Problem Type
Description
Solution
DNS records
Incorrectly published DKIM TXT record, typos, or wrong selector.
Double-check the DKIM selector name and public key value in your DNS. Ensure no extra spaces or hidden characters.
Email content
Email body or headers are modified after the DKIM signature is applied, leading to hash mismatch.
Configure sending systems to avoid modifying emails. Ensure canonicalization settings are compatible with RFC standards.
Alignment
The DKIM signing domain doesn't match the "From" domain, causing DMARC alignment failure.
Configure your ESP to sign with your sending domain or use a custom return-path domain that aligns. Ensure DMARC is properly configured.
Views from the trenches
Best practices
Always use a robust testing methodology, sending emails to various mailbox providers to check DKIM authentication.
Prioritize fixing issues with the primary DKIM key before attempting to implement or troubleshoot additional signatures for double DKIM.
Regularly monitor your DMARC reports for DKIM failure trends, which can highlight issues across different recipients or services.
Keep DNS records clean and organized, with clear naming conventions for DKIM selectors to avoid confusion.
Common pitfalls
Overlooking subtle typos or hidden characters in DNS TXT records, which can lead to frustrating and hard-to-diagnose failures.
Assuming that if DKIM passes for some recipients, it's working perfectly for all, neglecting to check specific mailbox providers like Yahoo.
Not understanding the impact of email content modification by third-party services or MTAs on DKIM body hash validation.
Ignoring the specific error messages in email headers, which often provide direct clues to the nature of the DKIM authentication problem.
Expert tips
Google's mail system is a reliable benchmark for checking email authentication due to its robustness and clear results, especially when troubleshooting Microsoft-related DKIM issues.
If DKIM issues are localized to a single recipient or a specific email server, consider potential caching issues on the recipient's end or unique server configurations.
When dealing with multiple DKIM keys, ensure that each key is meticulously managed, especially during migrations or updates to prevent conflicts.
Automate monitoring of DKIM validation status to detect failures promptly and address them before they significantly impact deliverability.
Expert view
Expert from Email Geeks says Google is a reliable benchmark for checking email authentication, as their system is robust and provides clear results. Microsoft has historically had issues breaking authentication, so if a problem occurs there, it is wise to first check with Google to determine if it is a broader issue.
2022-11-18 - Email Geeks
Expert view
Expert from Email Geeks says if an email header shows issues with the first DKIM key, it is advisable to prioritize fixing that initial key before attempting to add a second one, and to also check the Mail Transfer Agent (MTA) configuration.
2022-11-29 - Email Geeks
Maintaining healthy email authentication
Navigating the complexities of double DKIM implementation can be challenging, but understanding the common pitfalls and employing systematic troubleshooting methods can help resolve most errors. From precise DNS configurations to vigilant monitoring of email headers, each step is vital in ensuring your messages are authenticated correctly.
Consistently validating your DKIM records and keeping an eye on how different mailbox providers handle your authenticated emails will contribute significantly to your deliverability success and protect your sending reputation. For more technical details on the DKIM specification, you can refer to RFC 6376, the official DKIM standard.
