ARC – Authenticated Received Chain – is a way for email forwarders to mitigate the problems caused by users sending mail from domains with DMARC p=reject.
It allows a forwarder to record the DKIM authentication as they receive a mail, then “tunnel” that authentication on to the final recipient. If the final recipient trusts the forwarder, then they can also trust the tunneled DKIM authentication, and allow the mail to be delivered despite the DMARC p=reject published by the sending domain.
The specification and interoperability testing are progressing nicely and it’s definitely going to be useful for discussion list operators and vanity forwarders soon. It’s not something that’s as likely to help ESPs targeting small organizations and individuals, so all y’all shouldn’t be holding your breath for that.
There’s a more information about it at arc-spec.org and they’ve just published a great presentation with a technical overview of how it works:
(If the embedded presentation above is blank, try reloading the page or read it directly on slideshare.)