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.
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.
