A solution that works for me
1. First I edited /etc/postfix/main.cf file:
sudo nano /etc/postfix/main.cf
Comment: smtpd_milters = inet:localhost:12768
to become: #smtpd_milters = inet:localhost:12768
2. Second I edited /etc/postfix/master.cf file:
sudo nano /etc/postfix/master.cf...