Dovecot LMTP Proxy + STARTTLS?

Hi

I got dovecot 2.2.26 on a Centos7 with latest updates. Dovecot is configured to act as director and delivers to my two backend servers. I enabled lmtp proxy on director to listen on port 24.

Now I see in msg headers that the connection to the lmtp proxy uses STARTTLS but the connection from proxy to backend seems to be unencrypted. Is it possible to enforce the use of STARTTLS in the connection from the director to the backend as well?

Regards

tobi