This will create an email and send it to your email client. If you see the outgoing email, then you're all good.
List of mailto browser extensions
If you're accessing your email in a browser, you're using webmail and need a browser extension (a plugin) so that the outgoing emails open in your browser. There are generic plugins or proprietary plugins, for instance Mailto for Gmail if you're using Gmail.
Test Email Subject
Great, the mailto protocol works and you're good to go. Good luck.
If you're using an installed program such as Outlook or Thunderbird and the test email didn't open then you need to configure your operating system so that it connects your email program with the mailto protocol.