It is not a bug report. I am deploying a new ISPConfig 3.1.13p1 on an Ubuntu 18.04 VM. Postfix runs very well. Later, I found this comment in file /etc/roundcube/config.inc.php - // SMTP port (default is 25; use 587 for STARTTLS or 465 for the deprecated SSL over SMTP (aka SMTPS)) $config['smtp_port'] = 25; Then, I check my own netstat and it shows that postfix has all of them: 0.0.0.0:25 0.0.0.0:465 0.0.0.0:587 Now, the problem is - which mode of SMTP should we use when we configure a "send-mail" service? Should we disable TCP port 465 mode?