What is ARC (Authenticated Received Chain)?

ARC is a way for an intermediate mail server to vouch for the authentication results it saw before it modified or forwarded a message:

• It exists because forwarding breaks SPF - mailing lists and forwarders legitimately fail otherwise.
• It preserves the original verdict - the receiver can see that the message authenticated correctly one hop earlier.
• Trust is still the receiver’s choice - ARC provides evidence, not an entitlement.

You do not configure ARC yourself; your mail provider does. It is worth knowing why forwarded mail still arrives.