It looks like the SuperForms 2 stack just uses PHP’s mail function to send mail. We’ve had to disable that on our server environment as it’s outdated and unsecure (it allows for Spam attacks). A lot of web hosts have disabled this function for the same reasons.
For contact forms to work on our hosting platform, you’d need a Form stack that uses SMTP authentication. Our built-in Elements Contact Form component offers SMTP authentication, however I understand you are still on Classic.
If you are looking for a free alternative so you don’t have to spend more money on a stack, we are recommending https://letterbird.co/ as they give you a free form which you can embed on your Contact page and it works just fine on our platform.
If you need any help setting it up let me know, happy to help.