ARC
Authenticated Received Chain: an email header that preserves SPF, DKIM, and DMARC results across forwarding hops.
ARC (Authenticated Received Chain) solves a long-standing problem with email authentication: when a message is forwarded (through a mailing list, alias, or security gateway), the forwarder often rewrites the message in ways that break SPF and DKIM. ARC lets each intermediary cryptographically sign the authentication results it observed and pass that signed chain along. The final recipient can then trust the original verdict even if the message arrives from a different source. Without ARC, legitimate forwarded mail often fails DMARC and lands in spam.