Unlike NPrinting 16, version 17.x does not have an option to test the entered SMTP settings. You could run the attached PowerShell script to test your SMTP settings before entering them into NPrinting. You will need to make minor changes like specifying the SMTP server address & port number etc. The lines that need changing are listed in the comments at the beginning of the script.
You may need to set the execution policy to Unrestricted. To do that, first open Windows PowerShell Modules: