> Your mail either needs to be encrypted reliably against real adversaries or it doesn't.

It is, GPG take care of that.

> If the idea here is, a private group of friends can just agree never to put anything in their subjects, or to accidentally send unencrypted replies

That’s not what I’m talking about. It’s an enterprise - you cannot send non-encrypted emails from your work mail account, the gateway takes care of it. It has many rules, including such based on the sender and recipient.

Surely, someone can print the mail and carry it out of the company’s premises, but at this point it’s intentional and the cat’s already out of the bag.