Exchange: Online – Tag for external email
Exchange will make available a PowerShell cmdlet so that admins can enable the protections against spam & phishing threats from external senders. This will be achieved by presenting a new tag on emails called “External” in the message list in Outlook on the web, the new Outlook for Mac, Outlook for iOS and Android. In some Outlook clients, a “mail tip” will be included at the top of the reading pane with sender’s email address.
More info: https://techcommunity.microsoft.com/t5/exchange-team-blog/native-external-sender-callouts-on-email-in-outlook/ba-p/2250098