warning: TLS library problem: error:140760FC:SSL routines:SSL23_GET_CLIENT_HELLO:unknown protocol:s23_srvr.c:%:
Wojtek
Someone tries to use TLSv1 protocol but is not enabled.
You can enable TLSv1 in postfix and dovecot but such configuration is not recommended as it is not secure.
Apply it at your own risk otherwise ignore this error.