We know that some of our customers leverage Exchange transport rules to prepend subject line or insert the message body to show the email is from external senders. This approach has a few limitations which we heard:
We have heard the feedback on this, and are working on providing a native experience to identify emails from senders outside your organizations (which can help protect against spam & phishing threats). This is achieved by presenting a new tag on emails called “External” (the string is localized based on your client language setting) and exposing related user interface at the top of your message reading view to see and verify the real sender's email address.
If you are using the prepend subject line transport rules currently to add an [EXTERNAL] tag in external email subject line: the new Outlook native callouts are adding a new MAPI property called IsExternalSender to the email item. Once all the (above listed) client versions you require have this functionality, to avoid emails being marked ‘External’ twice (once by new native functionality and once by the transport rule), please turn off the transport rule first before turning on Outlook native external sender callouts.
We are tracking this feature in Microsoft 365 Roadmap ID 70595. This feature can be enabled on the tenant level now.
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:
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:
Once this feature is enabled via PowerShell, it might take 24-48 hours for your users to start seeing the External sender tag in email messages received from external sources (outside of your organization), providing their Outlook version supports it.
If enabling this, you might want to notify your users about the new feature and update your training and documentation, as appropriate.
Let us know here if you have any feedback!
The Outlook Team
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