Yes, absolutely. Authenticated Received Chain (ARC) is specifically designed to provide a verifiable chain of custody for an email as it travels from the original sender to the final recipient, even when it passes through intermediary servers like mailing lists or forwarding services.
At its core, ARC was created to solve a common problem that plagues email authentication protocols like SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail). When an email is forwarded, these authentication methods can break, leading to legitimate emails failing DMARC (Domain-based Message Authentication, Reporting, and Conformance) checks and potentially being marked as spam or rejected.
How forwarding breaks standard authentication
To understand why ARC is so important, we first need to look at why forwarding creates such a headache for email deliverability. Both SPF and DKIM have limitations when a message isn't delivered directly.
- SPF Failure: SPF works by checking if the IP address sending the email is authorized in the sender's DNS records. When a mailing list or forwarder resends your email, it does so from its own servers and its own IP address. The final recipient's server sees this new IP, which is not listed in your original SPF record, causing the SPF check to fail.
- DKIM Failure: DKIM creates a digital signature based on the content of the email. Many forwarding services, particularly mailing lists, add a footer or prefix to the subject line (e.g., [MyList] Your Subject). These modifications alter the message content, which invalidates the original DKIM signature.
Since DMARC relies on at least one of these checks passing and aligning with the 'From' domain, a forwarded email often fails DMARC validation. This is the exact problem ARC was engineered to solve.
How ARC creates a verifiable path
ARC works by adding a new layer of authentication headers to an email every time it's processed by an intermediary server. Think of it like a passport getting a new stamp at every border crossing. This process preserves the initial, valid authentication results.
With ARC enabled, each intermediary server adds its own cryptographic signature to the message header. This creates a sequence that allows the final receiving server to verify the entire chain of custody.
The ARC process involves three key headers:
- ARC-Authentication-Results (AAR): The first server in the chain evaluates the original email and records the SPF, DKIM, and DMARC results in this header. It essentially says, "When this email got to me, it passed authentication."
- ARC-Message-Signature (AMS): This is a new DKIM-like signature that covers the entire message, including any changes the intermediary made (like adding a list footer).
- ARC-Seal (AS): This is a signature that covers the previous ARC headers. It "seals" the entry for that specific hop, ensuring that the recorded authentication results and message signature haven't been tampered with.
When the next server in the chain receives the email, it validates the previous ARC-Seal and ARC-Message-Signature. Then, it adds its own set of AAR, AMS, and AS headers, creating another link in the chain.
Verification at the final destination
When the email finally arrives at its destination, the recipient's mail server will see that the direct SPF and DKIM checks fail, as expected. However, it will also see the chain of ARC headers.
The server can then validate the entire ARC chain, starting from the most recent ARC-Seal and working its way backward. If the entire chain is cryptographically valid, the server can trust the results recorded in the very first ARC-Authentication-Results header. This tells the server that the message was legitimate when it was originally sent.
So, to answer the question directly: ARC not only allows a recipient to verify the forwarding path but is the only standardized way to do so reliably. It provides the necessary context for a receiving server to trust a forwarded email, preserving the integrity of the original authentication and ensuring your message gets delivered.
