Microsoft is adding support for external email message tags to its cloud-based email service Exchange Online. The feature will only be available to Office 365 tenants later this month. Microsoft is also working on adding SMTP MTA Strict Transport Security (MTA-STS) support to Exchange Online to thwart downgrade and man-in-the-middle attacks. The new external email tags will only show up in Outlook on the web, the new Outlook for Mac, and Outlook mobile (iOS and Android)
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-365-adds-external-email-tags-for-increased-security/