In most forms, input in the email address field is a unique value that occurs only once in all submissions. That's why we generate a new unique email address for each of our tests, so that we always test your form as a "new user".
In order for us to know which of your fields is the email address field, you can set it in step 2 of the setup process.
Screenshot Formspector: Set the email address field during the setup process.