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.