How does email forwarding affect SPF, DKIM, and DMARC validation?

Key findings

• SPF breaks: When an email is forwarded, the forwarding server typically uses its own IP address to send the email to the next hop. SPF (Sender Policy Framework) checks the IP address of the sending server against the SPF record of the domain in the Return-Path header. Since the forwarding server's IP is unlikely to be authorized by the original sender's SPF record, SPF authentication almost always fails for forwarded emails.
• DKIM often survives: DKIM (DomainKeys Identified Mail) uses cryptographic signatures based on the email's headers and body. If the forwarding service does not alter the signed parts of the email (headers or body), the DKIM signature can remain valid. However, even minor modifications, such as adding a footer or changing specific headers, can break DKIM validation. Many forwarding services try to preserve DKIM to ensure deliverability.
• DMARC reliance on DKIM: DMARC (Domain-based Message Authentication, Reporting, and Conformance) requires either SPF or DKIM to pass with alignment. Since SPF typically breaks with forwarding, DMARC validation for forwarded emails heavily relies on DKIM passing and aligning. If DKIM also breaks, the email will fail DMARC.
• Spoofing vs. forwarding: A spike in DMARC rejections from unknown IPs, where DKIM/SPF/H.From appear to align with your domain, might indicate either forwarded emails or malicious spoofing attempts. The key differentiator is often the absence of corresponding bounce messages or customer complaints, which are typically seen with legitimate rejections (e.g., due to a DMARC reject policy). If there are no bounces, it's more likely spoofing where the recipient's server is correctly applying your DMARC policy.

Key considerations

• DMARC reporting analysis: Carefully analyze your DMARC aggregate reports to distinguish between legitimate forwarding failures and actual spoofing attempts. Look for patterns in source IPs, sending domains, and whether DKIM passes. A sudden, prolonged trend of rejections from an unrecognized IP range could suggest significant forwarding activity or a persistent spoofing campaign.
• Microsoft's behavior: Microsoft (and other large mailbox providers) may sometimes modify email bodies or headers during forwarding, which can invalidate DKIM signatures. This can lead to DMARC failures even for legitimate forwarded emails, especially if you have a DMARC policy of p=reject or p=quarantine. While mailbox providers generally try to handle forwarding gracefully, their internal processes can sometimes cause issues. For more details on this, you can refer to an article on email forwarding on DMARC.

Key opinions

• Forwarding causes SPF issues: Many marketers confirm that forwarded emails will almost certainly show different SPF domains and sender IPs than the original sends, leading to SPF failures. This is a widely accepted consequence of how SPF works with mail relays.
• DKIM is the lifeline for DMARC: For DMARC to pass on forwarded emails, it's essential for the DKIM signature to remain intact and align with the domain. If DKIM also fails, likely due to alterations by the forwarding server, then DMARC will fail.
• Distinguishing spoofing from forwarding: A common challenge is deciphering DMARC reports to determine if unknown IPs causing rejections are due to legitimate forwarding or malicious spoofing. The absence of elevated bounce rates or customer complaints about non-delivery often points towards spoofing attempts being correctly blocked by DMARC policies.
• Microsoft's impact: Concerns are frequently raised about how services like Microsoft's Outlook might alter email content or headers during forwarding, potentially breaking DKIM and causing DMARC failures for legitimate forwarded emails, even if a DMARC reject policy is in place. Marketers want to ensure their emails reach recipients even through forwarding chains.

Key considerations

• DMARC report limitations: Marketers often find their DMARC reporting tools inadequate for clearly identifying the root cause of failures, especially when distinguishing between forwarding and spoofing. This highlights the need for more granular and user-friendly DMARC analytics.
• Impact on transactional emails: The potential for transactional emails to be rejected due to forwarding issues is a significant concern, as it directly impacts customer experience and business operations. Ensuring deliverability for critical communications is paramount. You might consider how DMARC policies affect internal email deliverability.
• Understanding spoofing trends: A prolonged trend of DMARC rejections, even without bounces, suggests consistent spoofing attempts against the domain. Marketers should be aware that this is normal behavior for DMARC protecting their brand and doesn't necessarily indicate a problem with their legitimate sending. Understanding how SPF, DKIM, and DMARC work together is key.

Key opinions

• SPF will break: Experts universally agree that SPF validation almost always fails when an email is forwarded because the IP address of the forwarding server replaces the original sender's IP in the Return-Path header, leading to an SPF mismatch. This is a fundamental aspect of how SPF works.
• DKIM's resilience is key: DKIM is generally designed to survive forwarding as long as the signed parts of the message (headers and body) are not altered. If the forwarding agent modifies the email, DKIM will break. Therefore, for forwarded messages to pass DMARC, a valid DKIM signature is crucial. Further details can be found in discussions around DMARC passing when SPF fails.
• DMARC relies on DKIM for forwarded mail: Because SPF is compromised during forwarding, DMARC's success hinges entirely on the DKIM authentication passing and aligning with the From domain. If both SPF and DKIM fail, the DMARC policy will be applied, potentially leading to rejection.
• Interpreting DMARC reports: Experts advise that a consistent pattern of DMARC failures from unknown IPs, without corresponding bounce messages, often indicates successful blocking of spoofed mail. It's a sign that DMARC is performing its intended function of protecting your domain's reputation. Detailed analysis of DMARC reports is essential.

Key considerations

• Forwarding server behavior: The way a forwarding server handles an email (e.g., modifying headers, adding disclaimers, altering body content) directly affects DKIM validation. Some providers, like Microsoft, are known to make such alterations, which can unexpectedly break DKIM for forwarded emails. It's important to understand the nuances of DMARC enforcement with DKIM issues.
• Relaxed DMARC policy: While moving to a p=reject DMARC policy is recommended for security, experts advise caution, particularly when dealing with forwarded emails. For certain scenarios (e.g., internal forwarding, mailing lists), a p=none or p=quarantine policy might be necessary during the transition phase, especially if you anticipate issues with forwarded mail.
• Addressing legitimate forwarding: If you are concerned about legitimate forwarded emails failing DMARC, implementing Sender Rewriting Scheme (SRS) at the forwarding server level can help preserve SPF validation by rewriting the Return-Path. However, this is a solution for the forwarding party, not the original sender. More about SPF breaking for forwarded emails can be found on AutoSPF's blog.

Key findings

• SPF validation fails with forwarding: Documentation confirms that SPF authentication will fail when an email is forwarded, as the forwarding server's IP address will not match the original sender's authorized IP range in the SPF record. This is a standard and expected behavior of SPF.
• DKIM can survive forwarding: Technical sources indicate that forwarded emails can maintain DKIM validity if the forwarding service does not alter the signed headers or body of the message. This makes DKIM a more robust authentication method for messages undergoing forwarding compared to SPF.
• DMARC relies on DKIM for forwarded mail: Since SPF almost always breaks, the DMARC validation mechanism for forwarded emails typically relies on the DKIM signature passing and aligning. This is the primary way forwarded emails pass DMARC checks. For further reading, check out this article on how email forwarding passes DMARC validation.

Key considerations

• Content modification impact: Documentation warns that if an intermediate server or forwarding service alters the email's content or headers (e.g., adding disclaimers, altering attachments), it can break the original cryptographic DKIM signature, leading to DMARC failure. This is a known challenge to maintaining full authentication through forwarding chains.
• DMARC's role in spoofing defense: DMARC policies are designed to tell mailbox providers what to do with incoming emails that fail SPF and/or DKIM authentication, including those that might appear to be spoofing attempts. This robust defense mechanism can sometimes interfere with legitimate email forwarding, which is why monitoring DMARC reports is essential. For more, refer to SPF, DKIM, DMARC, and Email Forwarding.
• Mitigation techniques: Some forwarding systems or mail exchangers use techniques like Sender Rewriting Scheme (SRS) to address SPF breakage during forwarding. While not directly related to DMARC, these mechanisms aim to ensure the deliverability of legitimate forwarded emails.