You receive an error like "error setting certificate verify locations: CAfile: var/cw/systeam/cacert.pem CApath: /etc/ssl/certs" when trying to send a test email from the plugin's settings. This error is caused by a missing package on your web server. Please refer to this website for the solution: https://stackoverflow.com/questions/3160909/how-do-i-deal-with-certificates-using-curl-while-trying-to-access-an-https-url.
If you have a dedicated hosting, you or your system administrator should be able to install this package yourself. If you have a shared hosting, you will need to reach out to the support team of your hosting provider.