This error can occur if you have configured your SMTP Outgoing service to use a specific network interface (IP address), but then the server has changed IP address, leaving the interface unavailable.

To solve this problem, simply login as admin and click on Administration/Mail/SMTP Outgoing/General and select a suitable Network interface . Set it to 'Automatic' (which is the default setting) to let DeskNow pick the proper interface automatically, avoiding this problem in the future.