Hi Luboš,
we have a request for that, but not yet decided if we want to / have the resources to do so.
It is more complicated as it sounds, because currently messages are assembled on server-side and then streamed to the SMTP server. To allow encryption on client-side, the messages incl. it’s possible mime-parts / attachments or forwards need to be processed on client-side.
Are you interested in (co-)sponsoring that feature?
Ralf