You receive an error like "PHP Fatal error: Uncaught Error: Call to undefined function curl_init()" when trying to send a test email from the plugin's settings. This error is caused by a missing library on your web server: cURL. Please refer to this website for the solution: https://stackoverflow.com/questions/4477535/curl-init-function-not-working.
If you have a dedicated hosting, you or your system administrator should be able to install this library yourself (here is the documentation). If you have a shared hosting, you will need to reach out to the support team of your hosting provider.