Use the Email input type to collect an email address; the form will only accept values that match a standard email format- it does not actually validate the existence of an email address submitted. 

Although there is an option to use the User Email as Default, this feature generally won’t apply to your forms unless you have an intranet type or user-specific content workflow. 

In the validation section, you can check the Unique option if you want to limit submissions to one per email address.