The question of whether Authenticated Received Chain (ARC) ensures email sender authenticity is a nuanced one. At its core, ARC was developed not to establish initial sender authenticity, but to preserve the results of existing authentication protocols, like SPF and DKIM, through intermediaries. This becomes critical in scenarios where legitimate email forwarding or mailing list operations might otherwise break these authentications, leading to DMARC failures and potentially blocking legitimate emails.
Understanding ARC's function requires looking beyond a simple 'yes' or 'no' answer. It doesn't initiate the authentication process itself, but rather acts as a 'chain of custody' for authentication results. When an email server receives a message that has passed through a trusted intermediary, ARC provides a verifiable record of the original authentication status, helping recipient servers make informed delivery decisions, especially when DMARC is involved.
This mechanism is vital for ensuring that legitimate emails aren't incorrectly flagged as spam or rejected due to modifications made during transit. For senders, this means a higher likelihood of their messages reaching the inbox, even when they pass through multiple servers. For recipient mail systems, it provides additional context to trust email authentication, reducing false positives.
The challenge of email forwarding
The challenge of email forwarding
Traditional email authentication protocols, such as SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail), are highly effective at verifying the legitimacy of an email's origin. However, they were not designed to account for common email routing scenarios like forwarding or mailing lists. When an email is forwarded, intermediaries often alter parts of the message, such as headers or the 'From' address, in ways that break the original SPF or DKIM signature.
For instance, an email server acting as a forwarder might rewrite the 'Mail-From' address, causing an SPF check to fail because the forwarding server's IP address is not authorized by the original sender's SPF record. Similarly, mailing list software often adds footers or modifies content, invalidating the DKIM signature. These legitimate alterations lead to authentication failures at the final recipient, even for emails that were originally sent by an authenticated sender.
These failures become particularly problematic when a domain has a strong DMARC policy (p=quarantine or p=reject) in place. A DMARC record relies on SPF or DKIM alignment. When both fail due to forwarding, the DMARC policy dictates that the email should be treated with suspicion, potentially leading to it being rejected or sent to spam, despite being legitimate. This is where the Authenticated Received Chain (ARC) steps in to provide a solution for these authentication breaks. More details on this can be found in our guide on how to implement ARC.
How ARC preserves authentication results
How ARC preserves authentication results
ARC works by creating a 'chain of trust' among intermediaries. When an email passes through an ARC-enabled intermediary (like a forwarding service or mailing list), that intermediary takes a snapshot of the email's authentication results (SPF, DKIM, DMARC) at that point. It then cryptographically signs these results, along with relevant message headers, and attaches them to the email in the form of ARC headers.
This chain consists of three main headers for each hop: ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal. The ARC-Seal is particularly important as it cryptographically signs the previous ARC-Authentication-Results and ARC-Message-Signature headers, creating a verifiable link. Each subsequent ARC-enabled hop adds its own set of these headers, extending the chain.
When the email finally reaches the recipient's mail server, the server can then validate the ARC chain. If the chain is intact and signed by trusted intermediaries, the recipient mail server can choose to honor the original authentication results, even if the current SPF or DKIM checks fail. This mechanism helps reduce false positives, ensuring that legitimate emails are delivered as intended. For more in-depth information, you can refer to Microsoft's documentation on configuring ARC sealers.
ARC's interaction with DMARC for deliverability
ARC's interaction with DMARC for deliverability
ARC's primary benefit lies in its ability to give DMARC a 'second chance' for legitimate forwarded emails. When a DMARC-enabled recipient server receives an email, it first checks SPF and DKIM. If these fail due to forwarding, the server then looks for ARC headers. If a valid ARC chain is found, and the last ARC-Seal is from a trusted intermediary, the recipient server can use the original authentication results captured in the ARC-Authentication-Results header to pass DMARC.
The DMARC 'second chance'
ARC doesn't directly authenticate the sender but ensures that original authentication results from SPF and DKIM are reliably passed along, even after email modifications by intermediaries. This allows DMARC policies to correctly identify legitimate emails that would otherwise fail, improving email deliverability and security without replacing DMARC itself.
This crucial functionality means that ARC helps prevent legitimate emails from being incorrectly blocked, thereby improving overall email deliverability. Without ARC, many emails sent through mailing lists or forwarding services would consistently fail DMARC checks, leading to significant disruption for senders. It essentially allows for a more flexible interpretation of SPF and DKIM results in complex routing scenarios.
For organizations leveraging DMARC to protect their domain from spoofing and phishing, ARC is a valuable complement. It ensures that while enforcing a strong DMARC policy, you don't inadvertently impact the deliverability of your legitimate emails when they interact with third-party services that modify message headers. Tools like Suped's DMARC monitoring platform provide clear insights into ARC validation results within your DMARC reports, helping you understand its impact on your email streams and troubleshoot potential issues with DKIM implementation.
ARC and sender authenticity: a deeper look
ARC and sender authenticity: a deeper look
To directly answer the question: ARC itself does not establish sender authenticity. It does not validate the 'From' address nor does it re-authenticate an email. Instead, it creates a verifiable record of the original authentication status as the email traverses different servers. This record allows recipient mail servers to make more informed decisions when the direct SPF or DKIM checks would otherwise fail due to legitimate modifications made by intermediaries, such as mailing lists or forwarding services.
Therefore, while ARC doesn't perform the initial authentication, it plays a critical role in preserving the signals of authenticity. It acts as a bridge, ensuring that the valuable work done by SPF and DKIM is not undone by legitimate email flows. Without ARC, many genuine emails would be incorrectly flagged as potential spoofing attempts, even though they originated from authenticated senders. This is why ARC is considered an enhancement to, rather than a replacement for, existing authentication standards. You can read more about ARC's function in preserving authentication results across intermediaries on the Postmark blog.
Direct authentication protocols
SPF: Authorizes sending IP addresses for a domain, verifying the envelope 'Mail-From' address.
DKIM: Uses cryptographic signatures to verify message integrity and sender identity, linking the email to a domain.
DMARC: Builds on SPF and DKIM by checking alignment of the 'From' header, and specifies policy for unauthenticated mail.
ARC's role in preserving authentication
Chain of trust: Records and signs the authentication results as an email passes through intermediaries.
Integrity check: Allows recipient servers to verify the authenticity of past authentication steps.
DMARC assistance: Helps DMARC pass legitimate emails that would otherwise fail due to forwarding-induced modifications.
Ultimately, ARC is a crucial component in today's complex email ecosystem, working alongside SPF, DKIM, and DMARC to ensure that emails from legitimate senders reach their intended recipients, even when they traverse a path that would typically break traditional authentication checks.
Conclusion
Conclusion
ARC does not, by itself, ensure email sender authenticity in the same way that SPF or DKIM do for the original sending domain. Instead, its critical role is to preserve and communicate the results of those initial authentication checks as an email passes through various intermediaries. This 'chain of custody' ensures that legitimate emails are not unfairly penalized by DMARC policies simply because they were forwarded or sent via a mailing list.
By understanding ARC, email senders and administrators can better navigate the complexities of email deliverability, especially when dealing with third-party services. It's an essential layer that strengthens the overall email security framework, working in concert with existing protocols to build a more reliable and trusted email environment.
When the email finally reaches the recipient's mail server, the server can then validate the ARC chain. If the chain is intact and signed by trusted intermediaries, the recipient mail server can choose to honor the original authentication results, even if the current SPF or DKIM checks fail. This mechanism helps reduce false positives, ensuring that legitimate emails are delivered as intended. For more in-depth information, you can refer to Microsoft's documentation on configuring ARC sealers.
For organizations leveraging DMARC to protect their domain from spoofing and phishing, ARC is a valuable complement. It ensures that while enforcing a strong DMARC policy, you don't inadvertently impact the deliverability of your legitimate emails when they interact with third-party services that modify message headers. Tools like Suped's DMARC monitoring platform provide clear insights into ARC validation results within your DMARC reports, helping you understand its impact on your email streams and troubleshoot potential issues with DKIM implementation.
Therefore, while ARC doesn't perform the initial authentication, it plays a critical role in preserving the signals of authenticity. It acts as a bridge, ensuring that the valuable work done by SPF and DKIM is not undone by legitimate email flows. Without ARC, many genuine emails would be incorrectly flagged as potential spoofing attempts, even though they originated from authenticated senders. This is why ARC is considered an enhancement to, rather than a replacement for, existing authentication standards. You can read more about ARC's function in preserving authentication results across intermediaries on the Postmark blog.
