Using Classic Outlook on Windows for personal email, calendar, and contact management
Just a comment for interest's sake...
Whilst I can understand the frustration, I am just going through Cyber Insurance renewals for this year, and both of the Insurance companies require us to have the "External" tag in place. If we cannot do it, we need to provide the reason we can't. I expect this will become a widespread feature going forward, I myself found this thread while looking for how to turn it on for our organization.
I'm sure we have all seen Spam with "Internal" sender names, but from external email addresses, I think this is primarily to remind people to think twice.