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:
You can end up with duplicate [External] tags in subject line if external users keep replying to the thread (some of our customers use customized solutions to remove the duplicates).
Adding things to subject line breaks Outlook conversation threading, as the subject line is modified, so messages no longer “belong” to the same conversation.
Changed subject (or message body) stays as a part of the message during reply or forward, which leads to confusion if the thread becomes internal.
There can be localization issues, as transport rules have no knowledge of client language that end-users are using.
Those additions might take a lot of space in the subject line, making it hard to preview the subject on smaller devices.
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.
To set this up
Exchange Online tenant admin will need to run the cmdlet Set-ExternalInOutlook to enable the new user interface for the whole tenant (this is available now); adding certain emails and domains to the allow list via the cmdlet is also possible.
Outlook on the web already supports this. Outlook Mobile (iOS & Android) and Outlook for Mac are rolling out this feature. Specific versions:
Outlook on the web: available now
Outlook for Windows: available in builds 16.0.13930.10000 and higher
Outlook mobile (iOS & Android): version 4.2111.0 and higher
New Outlook for Mac: version 16.47 and higher
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 for Windows view of External sender (note that the experience is slightly different from others below):
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.