The question of whether Authenticated Received Chain (ARC) validates the 'From' address isn't a simple yes or no. The relationship is more indirect. ARC doesn't validate the address itself, but it plays a crucial role in preserving the authentication results that do. To understand this, we first need to look at how standard email authentication, specifically DMARC, relies on the 'From' address.
The role of the 'From' address in DMARC
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is the protocol that ties everything together. Its primary job is to ensure that the domain in the 'From' address, the one your recipients see, is the actual sender. It achieves this by checking for "alignment" with two other protocols: SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail).
For DMARC to pass, an email must pass either SPF or DKIM, and the domain used in that passing protocol must align with the domain in the 'From' address. This direct link between the visible 'From' address and the underlying authentication is what makes DMARC so effective at preventing spoofing.
Why email forwarding breaks DMARC
The DMARC model works perfectly until an email is passed through an intermediary, such as a mailing list or a forwarding service. When you forward an email, the 'From' address remains the same, but the server sending the message changes. This new server is likely not listed in the original domain's SPF record, causing SPF to fail.
Similarly, some forwarding services add footers or subject prefixes (like "[EXTERNAL]") to emails. These modifications alter the message content, which breaks the DKIM signature. With both SPF and DKIM failing, the email fails its DMARC check, even though it was perfectly legitimate to begin with. This is the exact problem that ARC was designed to solve.
How ARC creates a chain of trust
When an email passes through an ARC-aware forwarding server, that server evaluates the message's initial authentication status. It then cryptographically signs these results and attaches them to the email in a new set of headers. This creates what is effectively a "chain of custody."
Each intermediary in the email's path can add its own ARC seal, creating a verifiable chain that traces the message's journey. When the final recipient's server receives the email and sees a DMARC failure, it can then check for a valid ARC chain. If the chain is intact and shows that the email originally passed DMARC, the server can choose to trust this information and override the local DMARC failure.
The final answer: ARC's relationship with the 'From' address
So, to come back to the original question: ARC does not validate the 'From' address directly. Instead, it preserves the integrity of the original authentication checks which were based on that 'From' address. It provides a way for a final recipient to look back in time and confirm that the message was authentic at the start of its journey, before forwarding broke the standard validation methods.
- ARC does not perform new validation. It doesn't check the 'From' address against the intermediary's servers.
- ARC preserves the original results. It securely captures the initial SPF, DKIM, and DMARC pass/fail status, which was dependent on the 'From' address.
- ARC provides context for DMARC failures. It gives receiving mail servers the information needed to differentiate between a maliciously spoofed email and a legitimate, forwarded email.
In essence, ARC acts as a trusted voucher. It doesn't re-validate your identity, but it carries a cryptographically signed note from a trusted party saying that your identity was already checked and approved. This makes it an essential protocol for ensuring deliverability in a world of mailing lists and complex email routing.
