Tagging "EXTERNAL" emails

Is it possible for MIAB to tag emails that originate from external servers? via subject line [EXTERNAL] or embed in the message body?

This link https://www.howtoforge.com/add-disclaimers-to-outgoing-emails-with-altermime-postfix-debian-etch shows how to do it via postfix hack. I am not keen on modifying MIAB that carries an imminent break with future upgrades.