ARC is an authentication mechanism that provides a verified "message chain" for messages and allows all intermediaries to handle an email between the source server in the recipient's mailbox to see which other entities have handled it before.
Enabling ARC-hosted mailboxes for Office 365 prevents email authentication results from failing due to modifications made by intermediaries, such as forwarding rules or mailing lists, before they reach the recipient's inbox. Thus, the results of authentication evaluation emails are maintained at each step during the delivery process between intermediaries.
"Email senders use authentication mechanisms such as SPF, DKIM, DMARC to authenticate emails, but some legitimate intermediary services may make changes to the email, which may cause the email to fail in the next step", Explains Microsoft on the Microsoft 365 road map.
"With this change, administrators will be able to add trusted intermediaries to the Microsoft 365 Defender portal to allow Microsoft to use these ARC signatures, thus allowing legitimate messages."
The ability to customize ARC configurations to include additional trusted intermediaries allows them to change messages and associate their signatures with their domain name, keeping ARC chains intact.
Microsoft estimates that this new feature will be generally available worldwide on web platforms next year, starting in March 2022.