Could you include a screenshot of what you are seeing?
External tagging can be implemented in multiple ways by your corporate email administrators or security team. A screenshot would really help to determine which method they have chosen to use.
The built-in method from Microsoft, is to include a single line in the Infobar. As you mentioned that it takes up a third of the available space in your Preview Pane, it sounds like they have implemented a method which actually modifies the message. In that case, you'll need to provide this feedback to your email administrators or security team, as that method is custom and not controlled by Microsoft from a user experience perspective (as your admins control the displayed message and the size of it).