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:
- The Outlook preview shows the warning message for each external email rather than the first line of the message like they are used to.
- A back and forth email conversation would have the warning prepended multiple times.
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.
