08-02-2022 08:51 AM
Exchange online allows you to mark external messages using a transport rule. The common way to do it is to prepend a Subject line or a message text with a warning string like
"[EXTERNAL] this email originated outside the organization".
This method has at least 2 disadvantages:
TagExternal does not change the subject line and body to mark a message but assigns a special category to a message. You can define this category (a text string) yourself.