$config['smtp_server'] = ''; $config['smtp_port'] = ''; $config['smtp_user'] = ''; $config['smtp_pass'] = '';This is better than commenting them out in config.inc.php or defaults.ini.php because defaults.ini.php may be updated to set them to something. Explicitly setting them to '' overrides that. By the way, I tried setting them to null and unset() but those didn't work.
Computer Tips - Roundcube: settings to use sendmail program instead of SMTP
Date: 2019feb7 Product: Roundcube Webmail Q. Roundcube: settings to use sendmail program instead of SMTP A. Roundcube will use the `sendmail` command to send user's mail if you have no smtp options set. This is often the easiest and best way to set it up. It works for me. In file /etc/roundcubemail/config.inc.php Set all smtp_ options to ''