Using Classic Outlook on Windows for personal email, calendar, and contact management
Cyber security is among other things about "Don't click links unless you 1) have to and 2) know it is safe", whether it is internal or external. Many people work in big companies where they do not know their coworkers. The same rules apply for internal and external e-mails. I might understand it in companies or environments where 95 % of the emails are internal. The taging should in the least be optional. Putting a tag on external email is a fake whitewashing of the internal ones.