Troubleshooting 5.1.9 DKIM rejection errors from t-online.de

Key findings

• Sporadic nature: The issue often appears sporadically and across entire sending networks, suggesting a potential instability or specific policy enforcement on t-online.de's side rather than a consistent sender-side misconfiguration.
• Provider specific: This rejection is frequently reported as unique to t-online.de, implying their DKIM validation might be more stringent or subject to unique conditions compared to other mailbox providers.
• No official guidance: There is often a lack of official announcements or clear postmaster responses from t-online.de regarding these specific 5.1.9 errors, making troubleshooting difficult.
• DMARC policy interaction: Even with relaxed DMARC policies for SPF/DKIM alignment, t-online.de may be implicitly requiring strict alignment, leading to rejection if not met.

Key considerations

• DKIM alignment: Ensure your DKIM alignment is set to strict, even if your DMARC record permits relaxed alignment. This can mitigate potential rejections from ISPs with stricter enforcement policies.
• Message content and encoding: Issues with email content encoding or non-standard SMTP practices can invalidate DKIM signatures. Review message formats for compliance.
• DNS stability: Transient DNS lookup failures at the receiving end can prevent DKIM public keys from being retrieved. Monitor your DNS health and ensure reliable DNS service for your DKIM records.
• Third-party sending services: If using a third-party sender like Mailgun, confirm they are signing emails correctly and that their DKIM configuration is robust. For more details on common DKIM failures, refer to common DKIM failure reasons.
• Troubleshooting methodology: When facing elusive deliverability issues, systematically rule out variables by performing tests, even if they seem unlikely to be the cause. More information on troubleshooting can be found in our guide on how to troubleshoot DKIM failures.

Key opinions

• Mailgun user experience: Some marketers using Mailgun have experienced this specific issue sporadically with t-online.de, indicating it's not isolated to a single sender or setup.
• Broad impact: The issue can affect an entire sending network simultaneously, irrespective of IP, domain, or content variations, suggesting a potential systematic issue on the receiver's side.
• Support advice: Even when DMARC allows for relaxed SPF/DKIM alignment, some support teams (like Mailgun's) advise trying strict domain alignment as a troubleshooting step, highlighting a potential hidden requirement from t-online.de.
• Testing challenges: Obtaining a t-online.de email address for testing can be difficult for non-customers, impeding direct troubleshooting efforts. This highlights the importance of having diverse test accounts, as discussed in troubleshooting DKIM failures across ISPs.

Key considerations

• Strict alignment tests: Consider implementing strict DKIM alignment temporarily to see if it resolves the issue with t-online.de, even if DMARC policy is relaxed. This can sometimes resolve DKIM failures at other major providers.
• Monitoring: Continuously monitor delivery to t-online.de for patterns, such as specific times of day or particular content types, even if no clear pattern is immediately apparent.
• Postmaster engagement: While challenging, persist in attempting to contact t-online.de's postmaster for insights into their specific DKIM validation rules or any ongoing network issues. For broader context on this, Spam Resource lists postmaster contacts.
• Isolate the variable: If you have access to a t-online.de mailbox, perform controlled tests by varying DKIM configurations, signing practices, and content to isolate the trigger for the rejection.

Key opinions

• DKIM checking failures: Experts suggest the issue might stem from failures in the recipient's DKIM checking mechanism rather than a problem with the sender's signature itself.
• Content encoding and non-SMTP sending: Issues like sending non-SMTP compliant emails or problems with message content encoding can invalidate DKIM signatures during transit or at the receiving server. This is a common cause for DKIM body hash verification failures.
• Transient DNS: Temporary DNS lookup failures can prevent the recipient's mail server from correctly retrieving the sender's public key, leading to a DKIM validation failure.
• Receiver responsibility: Based on available evidence, experts often conclude that such specific and intermittent issues are likely problems on the recipient's network (e.g., t-online.de's) rather than purely sender-side.

Key considerations

• Systematic ruling out: Even without a clear root cause, systematically ruling out all possible sender-side factors is crucial. This means testing various configurations and content variations.
• Debugging limited access: Troubleshooting issues on another network with no direct access requires methodical testing and careful analysis of bounce messages. This can be complex, as discussed in debugging DKIM for specific scenarios.
• DMARC reports: Utilize DMARC reports to gain insights into how t-online.de (and other receivers) are validating your emails. Even if they don't explicitly state 'strict alignment', the DMARC aggregate reports might reveal trends. Learn how to understand DMARC reports.
• Community knowledge: Engage with the deliverability community, as shared experiences can provide valuable clues or workarounds for specific ISP behaviors.

Key findings

• Signature integrity: RFC 6376 (DKIM) outlines that any modification to the signed parts of an email, whether headers or body, invalidates the DKIM signature, leading to rejection. This includes changes in whitespace, character encoding, or line endings.
• DNS lookup: The DKIM verification process relies on successfully querying DNS for the public key. DNS resolution failures, even temporary ones, will result in a DKIM check failure.
• Canonicalization: DKIM defines two canonicalization algorithms (simple and relaxed) for both header and body. Strict adherence to the chosen canonicalization is crucial, as mismatches can cause signature validation to fail.
• DMARC alignment modes: DMARC's relaxed and strict alignment modes determine how closely the From domain must match the SPF or DKIM signing domain. While relaxed is permissible, some ISPs may implicitly prefer or require strict.

Key considerations

• DKIM record validity: Ensure your DKIM DNS record (TXT record with the public key) is correctly published and accessible. Errors like no DKIM record found will immediately lead to rejection.
• Selector choice: The DKIM selector (`s=`) specified in the signature must correctly point to the published public key in DNS. Refer to a list of common DKIM selectors.
• Header and body signing: The 'h=' and 'bh=' tags in the DKIM signature specify which headers are signed and the body hash. Ensure these are correctly generated and match the email content. For more on this, consult RFC 6376 on DKIM specifications.
• Error codes: A 5.1.9 error is a permanent failure (permerror), indicating a hard rejection. This differs from a temporary error (temperror), which suggests a transient issue. Understanding the distinction is vital for debugging, as explained in decoding DKIM temperrors.