ARC (Authenticated Received Chain)

An email authentication protocol (RFC 8617) that preserves SPF, DKIM, and DMARC validation results across forwarding hops, solving the forwarding authentication breakage problem. Each forwarding server adds an ARC-Seal and ARC-Message-Signature header, creating a chain of trust back to the original sender. Receiving servers can evaluate the ARC chain when direct DKIM verification fails due to forwarding.