Email Respondent will enable for both: Before Login page form and After Login page form.
Before Login: If you have enabled the Email feature for the Before Login page as well, the portal users get an email with the form details to the email that is entered in the Email field of the form. So the Email field must be in the Form.
After Login: If you have enabled the Email feature, the portal users get an email with the form details to the email that they used for the portal logged in.