If every sent message cannot be delivered because of this error, it means that something is blocking DeskNow from generating outgoing SMTP traffic.

Most likely this error is caused by an antivirus software that is monitoring outgoing SMTP traffic and that allows only certain applications to send email outside.

Solution

Check for any antivirus software installed on the server or on the router, and make sure that the DeskNow application is allowd to send emails outside.