LMTPS : TLS over LMTP not working

Hello, I tried to activate SSL on LMTP service, to secure connections between Postfix and Dovecot on my LAN, but Dovecot is not negociating a TLS session with Postfix. If I enforce TLS for LMTP at Postfix's side, communication between Postfix and Dovecot is not working.

I put ssl = yes ssl_cert =

and service lmtp { inet_listener lmtp { name = dovecot_lmtp address = xx.xx.xx.xx port = 26 ssl = yes } process_min_avail = 5 } within 10-master.conf

Did I miss something ?

Thank you for your help.

Regards, Stan