Clicking unsubscribe isn't the only way to know if the email is good. If your email client loads images then it will be known if you opened the email which will also give your IP, geolocation, device type, client name, user agent, etc.

Depending on your email provider if you mark it as spam that could also be known by the sender if your provider supports the complaint feedback loop.

Here are the docs to see what info is available when sending through mailgun.

https://documentation.mailgun.com/en/latest/api-events.html#...