Authenticated Received Chain (ARC) is an email authentication system designed to allow an intermediate mail server like a mailing list or forwarding service to sign an email's original authentication results. This allows a receiving service to validate an email when the email's SPF and DKIM records are rendered invalid by an intermediate server's processing.
DMARC allows a sender's domain to indicate that their emails are protected by SPF and/or DKIM, and tells a receiving service what to do if neither of those authentication methods passes - such as to reject the message. However, a strict DMARC policy may block legitimate emails sent through a mailing list or forwarder, as the DKIM signature will be invalidated if the message is modified, such as by adding a subject tag or footer, and the SPF check will either fail (if the forwarder didn't change the bounce address) or be aligned with the mailing list domain and not with the message author's domain (unless the mailing list rewrites the From: header field.)
ARC was devised to solve this problem by giving intermediate servers a way to sign the original message's validation results. Even if the SPF and DKIM validation fail, the receiving service can choose to validate the ARC chain. If it indicates that the original message passed the SPF and DKIM checks, and the only modifications were made by intermediaries trusted by the receiving service, the receiving service may choose to accept the email. Validating an ARC chain only makes sense if the receiver trusts the ARC signers. In fact, an ARC chain can be counterfeited,[3] so ARC processing applies when receivers trust the good faith of ARC signers, but not so much their filtering practices.
