What Is ARC?
TL;DR
ARC, or Authenticated Received Chain, is a standard that lets an intermediary such as a mailing list record the authentication result it saw before it modified a message. A later receiver can read that chain and choose to trust the original result even though SPF and DKIM now fail.
Which headers does ARC add?
Three, added together as a set each time an ARC-aware intermediary handles the message. ARC-Authentication-Results records the SPF, DKIM and DMARC results as that hop saw them. ARC-Message-Signature signs the message much as a DKIM signature would, taking custodianship for the version being passed on. ARC-Seal then signs the chain itself so earlier entries cannot be edited.
Each set carries an instance number, i=1 for the first hop and upward, which is how a verifier reassembles the order. The chain is judged as a whole: once a seal fails to validate the chain is failed, and the earlier results carry no weight.
Why does forwarding break authentication without ARC?
A forwarder connects from its own servers, so SPF fails because that address is not in the original record. A mailing list usually also rewrites the subject or appends a footer, which changes the signed content and breaks DKIM. Both checks then fail, and a DMARC policy at reject tells the receiver to refuse a legitimate message.
ARC does not repair either check. It preserves the evidence that they passed one hop earlier, which is what allows the final receiver to reach a different decision.
Does ARC matter for cold email?
Directly, not much. ARC is implemented by the intermediaries in the middle, mailing lists, security gateways and forwarding services, rather than by the domain that originated the message, so there is nothing to publish in your DNS and no setting to switch on.
It matters when your mail gets forwarded, which happens more than most senders realise, because role addresses and old addresses often forward silently. The lever you actually control is DKIM, since a valid signature survives plain forwarding on its own.
Frequently asked questions
No. The specification is explicit that ARC is not a trust framework: a passing chain is extra information a receiver may feed into local policy, and a receiver is free to ignore it entirely. Its effect depends on which receivers choose to evaluate it.