arc=none

arc=none means the message had no Authenticated Received Chain when it reached the receiver. For most mail that is exactly what you would expect. ARC is added by intermediaries such as mailing lists, forwarders and filtering services. A message that went straight from the sender's mail system to your provider has nothing to add one.

What the RFC says

RFC 8617 §4.4 defines none as no chain present when the message arrived for validation. The RFC notes this is typical of mail received directly from the original sending server or from an intermediary that doesn't take part in ARC. In the validator's steps (§5.2), finding no ARC sets ends the process right away with none.

Why you may still see ARC headers

It is common to find arc=none and an ARC-Seal in the same message:

ARC-Seal: i=1; a=rsa-sha256; d=mail.example.org; s=arc-2026; cv=none; b=...
ARC-Authentication-Results: i=1; mx.example.org;
       spf=pass smtp.mailfrom=example.com; dkim=pass header.d=example.com;
       dmarc=pass header.from=example.com
Authentication-Results: mx.example.org;
       spf=pass smtp.mailfrom=example.com;
       dkim=pass header.d=example.com;
       dmarc=pass header.from=example.com;
       arc=none

That isn't a contradiction. The receiving provider found no chain on arrival (arc=none), then started one itself by sealing the message as instance 1. The first seal always carries cv=none (§4.1.3, §5.1), because there was nothing before it. Some large providers seal inbound mail this way so that the results survive if you later forward or filter the message.

When none matters

Only when something else went wrong. If a message failed DMARC because it passed through a forwarder or list on the way, ARC is the mechanism that could have saved it: a receiver may accept a failing message on the strength of a trusted intermediary's seal (§7.2.1), which shows up as arc=pass. With arc=none, that intermediary didn't seal, so the receiver had nothing to go on.

If you received this message

Nothing to worry about. Read the SPF, DKIM and DMARC results as usual. ARC adds evidence only when a message has been relayed.

If you run a forwarder or mailing list

If your subscribers' providers keep rejecting or spam-foldering list mail from domains with strict DMARC policies, add ARC sealing. Most current list managers and mail servers support it. Seal after you have made all your edits, and read the ARC guide for the setup.