Usually you'd enter the SMTP details in the email settings. Doesn't PHP mail or other non-SMTP setting work for you? If it's with the Hosting service, you'd need to sort it out with them
Are you actually sure you're using the correct SMTP details (of the hosting service or the email service you use such as Gmail or whatever)?
However this forum is not really a primary source of tech support (it's for user-to-user discussion (and assistance where possible)) but we can't access your hosting service, and probably qualified to do so either).
If you need help, log a ticket with Appnitro through your customer area. They can check the details you're using and make sure it's set up correctly.