A protocol that preserves email authentication results as a message passes through intermediary servers such as mailing lists or forwarding services. These intermediaries often break traditional SPF and DKIM checks. ARC solves this by having each intermediary add a signed set of headers — ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results — creating a verifiable chain of custody. Each set is numbered sequentially (i=1, i=2, etc.), and the cv= field indicates whether the prior chain validated successfully. ARC allows receiving servers to trust forwarded messages that would otherwise fail authentication.