As of 4/16, 2021 Outlook on the web, Mac, and mobile will display an External tag in the message list.

Outlook Desktop and OWA will show the sender's email address at reading pane info bar. Outlook mobile and Outlook for Mac will only see an external tag on the message reading pane, and users will need to click the tag to see the real sender’s email address.

Outlook on the web view of External sender:

thumbnail image 1 of blog post titled          
              
                                                                                                                  Native external sender callouts on email in Outlook                                                                                                      
                 
         
         

In Outlook for iOS, External sender user interface in the message list, External tag when reading chosen email and view of sender's email address after tapping External label:

thumbnail image 2 of blog post titled          
              
                                                                                                                  Native external sender callouts on email in Outlook