Troubleshooting 5.1.9 DKIM rejection errors from t-online.de
Michael Ko
Co-founder & CEO, Suped
Published 24 Apr 2025
Updated 18 Aug 2025
8 min read
Encountering a 5.1.9 DKIM rejection error specifically from t-online.de can be a frustrating experience. It often indicates that t-online.de is having trouble verifying your email's DKIM (DomainKeys Identified Mail) signature, which is a critical part of email authentication.
This error typically manifests as a bounce message stating something like "Missing, invalid or non-matching DKIM signature." While DKIM is a global standard, specific mailbox providers, especially large ones like t-online.de, can have their own nuances and stricter requirements for email authentication. This guide will walk you through diagnosing and resolving these specific rejections.
I've seen these issues frequently, and the key to resolution often lies in a methodical approach to verification and ensuring all components of your sending infrastructure are in perfect sync. This includes not just DKIM, but also considering its interplay with SPF and DMARC.
When you receive a 5.1.9 DKIM rejection from t-online.de, it indicates that the recipient server failed to validate your email's DKIM signature. DKIM is an email authentication method that uses cryptographic signatures to verify that an email was not altered in transit and was sent by the authorized sender. This helps prevent email spoofing and phishing.
The error message "Missing, invalid or non-matching DKIM signature" points to several potential issues. Either the DKIM record itself cannot be found in DNS, it's malformed, or the signature generated by the sending server doesn't match what the receiving server expects after processing the email content. T-Online.de is known for its strict adherence to authentication standards, meaning even minor discrepancies can lead to rejections.
The bounce message you receive should contain a diagnostic code that can provide further clues. Look for specifics like DKIM reject DKIMr, often followed by a number like (250). This confirms that DKIM authentication is the specific failure point for t-online.de.
Common causes of DKIM failures
Several factors can lead to DKIM failures. One of the most common is an incorrect or improperly published DKIM DNS record. This could be a typo in the record, an incorrect selector, or a public key that doesn't match the private key used by your sending server. Sometimes, DNS propagation delays can also temporarily cause issues, as it takes time for changes to propagate across the internet.
Another frequent culprit is email content modification during transit. If an email is altered after it's been DKIM-signed, the signature will no longer be valid. This can happen with mailing lists, email forwarding services, or even some security scanners that modify message headers or bodies. These modifications invalidate the cryptographic hash, leading to a DKIM authentication failure. For more details on common causes, you can read about why DKIM fails more broadly.
Finally, DKIM works in conjunction with SPF and DMARC. A misconfigured DMARC policy, especially if it requires strict alignment, can cause issues even if SPF or DKIM technically pass but don't align correctly with the From domain. While the 5.1.9 error specifically points to DKIM, ensuring proper DMARC, SPF, and DKIM configuration is always crucial for overall deliverability.
Valid DKIM record
A correctly formatted DKIM TXT record published in DNS, accessible by external servers. This record must exactly match the key used for signing.
Content integrity
Emails are signed correctly by your ESP/MTA and remain unaltered during transit to the recipient's server, ensuring the signature's validity.
Syntax errors or propagation issues
Mistakes in the DKIM TXT record, or recent DNS changes that haven't fully propagated, can lead to lookup failures.
Message modification
If any part of the email, including headers or body, is modified after DKIM signing, the signature will be invalidated, causing rejection.
Diagnosing the issue with t-online.de
Diagnosing a 5.1.9 DKIM rejection from t-online.de requires a systematic approach. Start by examining the full bounce message for specific details beyond the 5.1.9 code. Look for any additional information t-online.de might provide, such as which header or part of the signature failed.
I often find that issues with specific ISPs like t-online.de can be tricky because their internal policies are not always publicly detailed. While their postmaster guidelines are available, they might not explicitly cover every edge case leading to a 5.1.9 error. This is where meticulous testing and observation become vital. Sometimes, issues are transient, perhaps due to temporary DNS problems on their end or yours.
To effectively troubleshoot, send a test email to a t-online.de address and analyze the full email headers if it reaches the inbox, or the bounce message if it's rejected. You're essentially looking for clues in the authentication results section of the headers (or the diagnostic code in the bounce) to pinpoint where the DKIM validation failed. This method helps in troubleshooting DKIM failures with precision.
Important for diagnosing DKIM issues
When diagnosing these errors, ensure you have access to the complete bounce message, including all the technical details and diagnostic codes. This information is crucial for pinpointing the exact cause of the DKIM rejection. Also, consider the specific behaviors of t-online.de, as some providers have unique validation processes.
Steps to resolve 5.1.9 errors
The first step in resolving a 5.1.9 DKIM rejection is to thoroughly verify your DKIM DNS record. Ensure it's published correctly as a TXT record, your selector is accurate, and the public key exactly matches the one used by your email service provider (ESP) or mail transfer agent (MTA). Sometimes, a simple copy-paste error or an extra space can invalidate the record. If you're encountering issues like "DKIM record published no DKIM record found" errors, this step is particularly critical.
Next, focus on ensuring that your email content isn't being modified in transit. This is a common cause of DKIM body hash did not verify errors, which often lead to 5.1.9 rejections. Work with your ESP or IT team to confirm that no intermediate systems (like firewalls, proxies, or archiving solutions) are altering the email headers or body after it's been signed. If you're using a third-party sending service, ensure their DKIM implementation is robust and correctly aligned with your domain.
Finally, review your DMARC policy's alignment settings. While t-online.de might have a specific internal requirement, ensuring strict DMARC alignment for both SPF and DKIM can often mitigate these types of rejections. Even if your DMARC policy is set to p=none, a relaxed alignment can sometimes cause issues with very strict receivers. Regularly checking your DKIM success rate can help identify trends and address problems proactively.
Verify DNS: Double-check your DKIM TXT record for typos, correct selector, and proper key. Use an online DNS checker to confirm it's publicly accessible and resolving correctly.
Check email content: Ensure no intermediate systems are modifying your email content or headers after DKIM signing. This is particularly relevant for transactional emails that might pass through multiple systems.
Review DMARC alignment: While your DMARC policy might be relaxed, t-online.de might implicitly require strict alignment for better deliverability. Test with strict alignment if possible.
Engage your ESP/MTA provider: They can check their logs for specific DKIM signing failures or rejections from t-online.de, offering insights into transient issues or specific policy changes.
Best practices for DKIM
To prevent future 5.1.9 errors, ensure that you regularly monitor your DKIM authentication rates. Use DMARC reports to identify consistent failures and proactively address them. Maintain a simple email path where possible to minimize content modifications. For t-online.de specifically, consider sending test emails frequently to monitor their reception and identify any changes in their authentication requirements.
Views from the trenches
Best practices
Always ensure your DKIM DNS records are accurately published and routinely verified for correctness and propagation delays, which helps prevent authentication failures.
Minimize email content modifications post-signing by carefully configuring mailing lists and internal forwarding systems, as alterations invalidate DKIM signatures.
Implement a DMARC policy with strict alignment settings for SPF and DKIM, especially when facing rejections from providers known for strict authentication, such as T-Online.de.
Regularly review your DMARC reports and authentication statistics to proactively identify and resolve any underlying DKIM or SPF alignment issues before they impact deliverability.
Common pitfalls
Overlooking subtle typos or incorrect selectors in DKIM DNS records, leading to a 'no key for signature' error and failed authentication.
Failing to account for transient DNS lookup issues that can intermittently cause DKIM validation failures, which are difficult to diagnose without consistent monitoring.
Relying on relaxed DMARC alignment when sending to ISPs like T-Online.de, which may implicitly demand stricter adherence, resulting in unexpected rejections.
Not engaging your email service provider to investigate their signing logs or specific rejections from target domains, missing crucial diagnostic insights.
Expert tips
Maintain a robust feedback loop with your ESP to quickly detect and troubleshoot authentication failures reported by receiving mail servers.
When troubleshooting, isolate the issue by sending simple test emails with minimal content to see if the rejection persists.
Consider using a dedicated email deliverability monitoring service to track authentication rates and receive alerts for any dips.
For specific problematic ISPs like T-Online.de, try to establish direct contact with their postmaster team if other troubleshooting steps fail, though direct responses can be rare.
Marketer view
Marketer from Email Geeks says that the 5.1.9 DKIM rejection error sometimes occurs with Mailgun.
2025-07-03 - Email Geeks
Expert view
Expert from Email Geeks says that these issues might stem from DKIM checking failures, non-SMTP sending, or email content encoding problems.
2025-07-04 - Email Geeks
Conclusion
Dealing with 5.1.9 DKIM rejection errors from t-online.de can be challenging, but it's a fixable problem with diligent troubleshooting. By systematically checking your DKIM DNS record, ensuring message content integrity, and reviewing your DMARC alignment, you can significantly improve your email deliverability to t-online.de recipients. Remember that robust email authentication is key to maintaining a good sender reputation and ensuring your emails reach the inbox reliably.
