Each hop adds a set of three headers with the same instance number i= (1 to 50): ARC-Authentication-Results (the SPF, DKIM and DMARC results it observed), ARC-Message-Signature (a DKIM-like signature over the message) and ARC-Seal (a signature over the previous ARC headers, with cv=none, pass or fail describing the chain so far). A receiver that trusts a sealer can accept a message whose DMARC failed only because a known forwarder modified it.
ARC never left experimental status, and receivers decide case by case which sealers to trust. In April 2026 the IESG rechartered the DMARC working group to move RFC 8617 to historic status, as DKIM2's per-hop signatures aim to replace it. The ARC viewer lays out a message's chain.