Work around disabling stric TLS, allowing to old, not updated Exchange server (in my case) to connect to Postfix at Plesk:
In /etc/postfix/main.cf , comment lines and insert the last one, as above:
#smtpd_tls_ciphers = medium
#smtpd_tls_mandatory_ciphers = medium
#tls_medium_cipherlist =...