LMTPS : TLS over LMTP not working

[Stanislas SABATIER]

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 = </dovecot/ssl/ssl-LMTP.pem
  ssl_key = </dovecot/ssl/ssl-LMTP.key
in section protocol LMTP within 20-lmtp.conf

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