What are ARC headers (ARC-Seal, ARC-Message-Signature, ARC-Authentication-Results)?
ARC Authentication Results (AAR) records SPF, DKIM, and DMARC results from the previous hop in the RFC 8601 structure.
ARC Message Signature (AMS) signs the message body and selected headers for that hop.
ARC Seal (AS) signs both the AAR and AMS and links them to the full sequence of earlier ARC sets.
Together these form a cryptographic history of the message’s journey, with each hop adding a signed and traceable entry.
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