Recipient Can't receive Test Email Testing

We are currently testing the "Send Email" tab on Kentico (v12.0.103) running on WinServer 2016.

We noticed once we use test@gmail.com or test@outlook.com on the "From" area, the recipient "To" can receive the test email. While when we use test@kns.com((specifically this domain @kns.com), the test email was not received by the recipient. Also, when we try to use the "Generate new Password" tab, there was no email notification on the recipient.

Is there any setting to allow us to receive the email from @kns.com?