Symptoms

Since Plesk 8.3 the mail message submission port can be configured through Plesk CP to protect your server against unauthorized mail relaying or injection of unsolicited bulk mail.

Select the Enable message submission check box on the Server->Mail page in Plesk CP to allow your customers to send e-mail messages through the port 587. Enabling Mail Submission port does not affect usual 25 (SMTP) port, it is just additional way to send mail only from authorized users. Now you can set relaying mode to closed on the Server->Mail page in Plesk CP, so mail will only be received on 25 port, and notify your customers that they need to specify in their e-mail programs' settings the port 587 for outgoing SMTP connections, and be sure to allow connections to this port in your firewall settings.