The Authenticated Received Chain (ARC) was designed to address situations where email messages that are forwarded, and in some cases altered, will no longer pass email authentication checks. After spending 3 years as a work item in the IETF‘s DMARC Working Group, the specification was published as RFC 8617 (HTML, PDF, plain text) on July 9th, 2019. More information about ARC can be found at the website arc-spec.org.
Even though RFC 8617 has just been published, there are already a number of implementations. Google deployed it for their GMail, G Suite, and Google Groups services by 2018; AOL had a fully functional implementation under evaluation in 2017-2018; and Fastmail has incorporated ARC verification into their services. The commercial Mail Transfer Agents (MTA) Halon MTA and MailerQ already include support, and ARC sealing/signing is available in the Mailman and Sympa mailing list managers (MLM). There are also several open source libraries or modules that can be used to add ARC functionality to a product or package, as well as a full validation suite. Check this Resources page for more details.