Unable to send emails in Plesk with Imunify360: SMTP Error (-1): Connection to server failed

Symptoms

• Unable to send emails in Plesk. The following error is shown in webmail (Roundcube):

  SMTP Error (-1): Connection to server failed.

• An email sent to any external mailbox may be bounced back with the following error:

  <externalmailbox@example.com>: mail for example.com loops back to myself

• /var/log/plesk-roundcube/errors shows the following errors:

  PHP Warning: stream_socket_client(): unable to connect to ssl://localhost:465 (Connection refused) in /usr/share/psa-roundcube/vendor/pear/net_socket/Net/Socket.php on line 159
  PHP Error: Connection refused (POST /roundcube/?_task=mail&_unlock=loading1617593106533&_framed=1&_lang=en_US&_action=send)
  SMTP Error: Connection failed: Failed to connect socket: Connection refused in /usr/share/psa-roundcube/program/lib/Roundcube/rcube.php on line 1702 (POST /roundcube/?_task=mail&_unlock=loading1617593106533&_framed=1&_lang=en_US&_action=send)

Cause

Imunify360 firewall only allows SMTP connections from the root user or mail group.

Resolution

1. Log in to Plesk GUI

2. Go to Imunify360 > Settings ​ > SMTP Traffic Management > Configure the Allow users with the following users: postfix,horde_sysuser,roundcube_sysuser > Click Save changes to apply the configuration: