What is ARC?
ARC helps to preserve email authentication results and verifies the identity of email intermediaries that forward a message on to its final destination. There are three key components to ARC:
- ARC Authentication Results header: a header containing email authentication results like SPF, DKIM, and DMARC
- ARC Signature: a DKIM-like signature that takes a snapshot of the message header information, including the to, from, subject, and body
- ARC Seal: another DKIM-like signature that includes the ARC Signature and the ARC Authentication Results header information.
To read more about Authenticated Received Chain(ARC) please refer to the below link:
https://knowledge.progist.net/index.php/2021/05/19/authenticated-received-chain/
ARC headers:
- ARC-Authentication-Results (AAR) header: records the contents of the Authentication-Results header from DMARC. The purpose of these header is to check whether any previous authentication passed or failed.
- ARC-Message-Signature (AMS) header: includes a cryptographic signature of the message itself up to this point.
- ARC-Seal (AS) header: includes a cryptographic signature of the message headers.
All of the above explained headers together are called an ARC set. Multiple ARC sets – if present – represent an ARC chain. Each of the headers within an ARC set contains a tag “i=” which represents the sequence number starting from 1 and it is the same for each ARC header within a set.