Fix ARC Header and Validation Warnings

ARC helps preserve authentication context when mail is forwarded through intermediaries like mailing lists, gateways, or forwarding services.

What ARC warnings usually mean

  • ARC headers are completely absent
  • The ARC chain is incomplete
  • Instance numbers are malformed
  • Crypto verification failed on a signed chain

When to take action

  • Act when your mail is routinely forwarded before reaching recipients
  • Investigate when an intermediary is supposed to sign ARC but does not
  • Treat ARC as secondary if DKIM and DMARC are already healthy and no forwarders are involved

Practical checks

  1. Identify whether a forwarder or mailing list is in the path.
  2. Inspect ARC-Seal, ARC-Message-Signature, and ARC-Authentication-Results.
  3. Confirm the same system is adding a complete ARC set for each hop.
  4. Fix the forwarding service or gateway before changing your public DNS.

ARC is not a replacement for DKIM or DMARC. It is a forwarding aid, so fix the systems rewriting mail in transit.