How do I troubleshoot DMARC failures and potential DKIM replay attacks affecting email deliverability?

Key findings

• Normal forwarding behavior: When recipients programmatically forward messages (e.g., via filters), it typically breaks SPF authentication but not DKIM, as the DKIM signature often remains intact. This is a common and expected behavior, not necessarily an indication of a security breach.
• DMARC compliance with forwarding: Even with SPF failing, if DKIM remains valid and aligned, the email can still pass DMARC. This explains why some DMARC reports might show emails passing DMARC that were not directly sent by your infrastructure.
• Distinguishing issues: It is crucial to differentiate between legitimate forwarding patterns and actual DKIM replay attacks. Genuine replay attacks involve malicious actors reusing your valid DKIM signature to send fraudulent messages from your domain.
• Domain reputation impact: Unusual DMARC failure patterns, even if caused by forwarding, can sometimes correlate with dips in domain reputation, particularly at major inbox providers like Google.
• Rate limiting: Google Postmaster Tools may report rejections due to 'rate limit exceeded', which can sometimes be mistaken for DMARC issues but often indicates sending too quickly for a given domain's reputation or volume.

Key considerations

• Analyze DMARC aggregate reports: These reports provide the most reliable data to understand where and why DMARC failures are occurring. They help determine if rejections are due to policy enforcement or other issues. For detailed insights, refer to our guide on interpreting DMARC reports.
• Verify email authentication setup: Ensure your SPF and DKIM records are correctly configured to pass authentication and alignment checks. Misconfigurations are a leading cause of DMARC failures.
• Monitor domain reputation: Continuously check tools like Google Postmaster Tools for any shifts in domain or IP reputation, as this can indicate underlying deliverability issues, including those potentially related to DMARC. Learn more about troubleshooting DMARC failures.
• Review sending practices: If you are seeing 'rate limit exceeded' messages, assess your sending volume and speed. It might be too aggressive for the receiving server or your sending reputation.

Key opinions

• Forwarding confusion: Many marketers initially suspect a security breach, like a DKIM 'hack', when DMARC reports show emails passing authentication from unexpected sources, especially if SPF fails but DKIM passes. They often question if someone is replaying their DKIM signature.
• Reputation concerns: There's a strong belief that any unusual activity in DMARC reports or Google Postmaster Tools, such as 'rate limit exceeded' messages, directly impacts domain reputation and deliverability, even for low sending volumes.
• Unexpected DMARC passes: It can be perplexing to see a significant number of DMARC pass emails in the 'Forward' section of reports that were not sent by their own systems, raising questions about the source and legitimacy of these messages.
• Seeking quick fixes: Marketers often seek immediate solutions to unusual DMARC or deliverability issues, highlighting the need for clear troubleshooting steps and explanations.

Key considerations

• Verify sending patterns: Cross-reference DMARC reports with your own delivery logs and campaign metrics to identify discrepancies. This helps confirm whether the reported mail volume aligns with your actual sending activity.
• Understand DMARC report sections: Familiarize yourself with the different sections of DMARC reports, especially the 'Forward' section, and what they truly indicate. Not all DMARC passes for non-sent emails signify a problem.
• Consult Google Postmaster Tools: Use Postmaster Tools to get Google's perspective on your domain's reputation and deliverability issues, such as rate limiting. This can provide crucial context for DMARC failures. For more, see our ultimate guide to Google Postmaster Tools.
• Seek guidance on authentication: If DMARC is failing despite passing SPF and DKIM, or if you're experiencing alignment failures, investigate the causes. Our page on why DMARC authentication fails can provide further clarification. Also, be aware of situations like Outlook's new mystery rejection.

Key opinions

• Forwarding is normal: It is a normal and expected behavior for recipient servers to programmatically forward messages, which breaks SPF but not DKIM. This means DMARC can still pass due to DKIM alignment, even if your organization did not send the email.
• DKIM replay patterns: While forwarding is benign, experts confirm that certain patterns in DMARC reports, where SPF fails but DKIM passes for unauthorized mail, can indeed indicate a DKIM replay attack, especially if the volume is unusual or tied to specific suspicious IPs.
• Rate limiting vs. DMARC: Rejections due to 'rate limit exceeded' in Postmaster Tools are distinct from DMARC policy rejections. They typically suggest sending too fast, not an authentication failure. Confirming DMARC policy rejections requires reviewing DMARC aggregate reports.
• Importance of internal logs: Checking your own delivery logs (MTA logs) for similar rate limiting or rejection messages is critical for confirming external reports and identifying any changes in campaign metrics that might signal an issue.
• Strategic header signing: While not all headers need to be signed (e.g., received headers which change upon forwarding), signing additional headers like 'Cc' is generally not harmful and can provide an extra layer of protection, though typically not targeted in replay attacks.

Key considerations

• Dive into aggregate reports: Always consult your DMARC aggregate reports (RUAs) for definitive answers on DMARC policy rejections and to pinpoint sources of unauthorized mail. This is crucial for understanding and troubleshooting DMARC reports.
• Assess suspicious IPs: If a suspicious IP range appears in your DMARC reports, especially one known for questionable activity, investigate further as this could indicate a DKIM replay attack or other abuse.
• Implement preventive measures: If a DKIM replay attack is suspected, take steps to secure your sending. This often involves ensuring proper DKIM key rotation, stronger email authentication configurations, and monitoring. Refer to resources like this SocketLabs article on preventive measures. Also, understanding how to debug DMARC authentication failures is key.
• Continuously monitor and adapt: Email deliverability is dynamic. Regularly review your DMARC reports, Postmaster Tools data, and adjust your sending practices or authentication configurations as needed to maintain optimal deliverability and protect against evolving threats.

Key findings

• DMARC authentication flow: DMARC works by verifying that either the SPF or DKIM identifier aligns with the domain in the 'From' header. If neither aligns, the DMARC policy (none, quarantine, or reject) is applied.
• SPF and forwarding: SPF relies on the sending IP address. Email forwarding often changes the sending IP, causing SPF to break during subsequent authentication checks.
• DKIM and forwarding: DKIM uses cryptographic signatures applied to email headers and body. Unless the content or signed headers are modified during forwarding, the DKIM signature typically remains valid, allowing it to pass authentication.
• DKIM replay vulnerability: A DKIM replay attack can occur if an attacker obtains a valid DKIM-signed email and reuses its signature on a modified or entirely new message, aiming to spoof the legitimate sender's domain. This typically requires weaknesses in DKIM key management or insufficient header signing.
• Header canonicalization: DKIM signatures are sensitive to changes. Headers that are frequently altered by intermediaries (like Received- and Resent- headers) should generally not be included in the signed header list (h= tag) to prevent legitimate forwarding from invalidating the signature unnecessarily.

Key considerations

• Understand alignment requirements: DMARC requires either SPF or DKIM to align with the 'From' domain. This means the domain in the SPF HELO/MAIL FROM or the DKIM 'd=' tag must match the organizational domain in the 'From' header. Our simple guide to DMARC, SPF, and DKIM provides a great overview.
• Strategic DKIM header signing: To mitigate replay attacks while allowing legitimate forwarding, carefully select which headers are included in the DKIM signature (the 'h=' tag in the DKIM-Signature header). Avoid signing headers that are expected to change during transit. This helps DKIM records reduce email spoofing.
• Utilize DMARC policies: Gradually transition your DMARC policy from 'p=none' to 'p=quarantine' and eventually to 'p=reject' based on your DMARC reports. This allows you to monitor and adjust before enforcing strict policies that could block legitimate mail. For a list of DMARC tags and their meanings, refer to our guide on DMARC tags.
• Secure DKIM keys: Ensure your DKIM private keys are securely stored and regularly rotated to minimize the risk of compromise that could enable replay attacks.