SMTP Test is mostly useful for app development. Instead of configuring a real email account for sending emails, your application can use it for message delivery. You get the same experience as using any other email service provider, you can configure it as an outbound mail server and send your transactional or marketing emails through the SMTP Test service. Unlike real email services SMTP Test only accepts mail for delivery but never actually delivers anything. Instead, all the messages will be forwarded to your mailbox.

This guide will help you to whitelist your entire domain name: https://www.marottaonmoney.com/how-to-whitelist-an-entire-domain-in-gmail/

* In the "To" box type your domain name instead of the "From" box.

No, you can use the server without worrying about any limitations. However if the system detects any malicious activity, it will hold the account.