If you are using the prepend subject line transport rules currently to add an tag in external email subject line: the new Outlook native callouts are adding a new MAPI property called IsExternalSender to the email item. New Outlook for Mac: version 16.47 and higher.Outlook mobile (iOS & Android): version 4.2111.0 and higher.See Update history for Microsoft 365 Apps (listed by date) to see release status of Version 2211.

If that changes we will update this topic. We anticipate the External tag to reach other channels MEC, SAC, as Version 2211 advances. Outlook for Windows: Update 1/26/23: External Tag view in Outlook for Windows (matching other clients) released to production with Current Channel Version 2211 for builds 15831.20190 and higher.Outlook Mobile (iOS & Android) and Outlook for Mac are rolling out this feature. Outlook on the web already supports this.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.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. 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). Those additions might take a lot of space in the subject line, making it hard to preview the subject on smaller devices.There can be localization issues, as transport rules have no knowledge of client language that end-users are using.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.Adding things to subject line breaks Outlook conversation threading, as the subject line is modified, so messages no longer “belong” to the same conversation.You can end up with duplicate tags in subject line if external users keep replying to the thread (some of our customers use customized solutions to remove the duplicates).This approach has a few limitations which we heard: 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.

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