When you click the Share button, as shown in the screenshot below, it creates a unique share URL (e.g., https://mail.mixmax.com/m/5m3VxNmUJ75NZdxRa). The ID on the URL is random, so it's nearly impossible to guess. The shared email is available on a web page, and you can choose who is able to view the link and expiration date:

To stop the email from being shared, click the trash can icon (see screenshot below) - that deactivates the link, and no one can access it.