Authenticated Received Chain


Authenticated Received Chain 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.
ARC is defined in RFC 8617, published in July 2019, as "Experimental".

Overview

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 SPF check will fail due to the unapproved sender, and the DKIM signature will be invalidated if the message is modified, such as by adding a subject tag or footer.
ARC helps 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. If the ARC 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.

Implementation

ARC defines three new mail headers:
To sign a modification, an intermediate server performs the following steps:
To validate an ARC, the recipient performs the following steps: