With the adoption of stricter email authentication policies to curb spam, intermediate mail handlers like mailing lists suffer potential problems of getting their mails flagged as spam and returning undelivered. The solution for this lies in the recently drafted IETF ARC (Authenticated Received Chain) protocol. From Mailman's point of view, ARC is a protocol that can help mitigate denial of service to subscribed addresses at Yahoo!, AOL, and other sites that have a
p = reject DMARC policy. With Google too set to migrate to a DMARC
p=reject policy in the near future, the importance of the ARC protocol has become more significant.Basically, setting up ARC would allow Mailman to securely register its handling of the message, thus allowing the set-up of a trust mechanism between Mailman and the involved MTAs and hence reducing the denial of service.