Microsoft has enabled Authenticated Received Chain (ARC) for all for Office 365 hosted mailboxes to improve anti-spoofing detection and to check authentication results within Office 365. ARC is a protocol designed to provide an authenticated “chain of custody”” for messages. ARC supplements the DMARC and DKIM email authentication protocols as part of Internet Mail Handlers’ effort to combat email spoofing especially when dealing with forwarded messages. Microsoft also rolled out the new ‘Unverified Sender’ feature during October to make it easier for users to identify potential spam or phishing emails delivered to their Outlook clients’ inboxes.”
Source: https://www.bleepingcomputer.com/news/microsoft/office-365-enables-arc-for-enhanced-anti-spoofing-detection/