It would be a good idea to add an option to set / detect the email Sensitivity: header field. This is a RFC-standard field that is also settable in MS Outlook's Options dialog for example.
This'd be the natural hook to trigger an external security solution, as in, set Sensitivity to higher than normal in the response generation to get encryption for outgoing mails. (In case you're discussing potentially sensitive or personal information, which needs to be protected in transit according to GDPR.)
The part about encryption triggered by Sensitivity header I already tested and found working with our mail server, now we just need to have the trigger for it. A keyword in the text field is just ugly.