Best Wordpress plugin to engage your users and collect their email. You can use header alerts, popups, or exit pages, and it works with any popular emailing service such as Mailchimp or AWeber
WPS only autofill the email address with user that are logged into the wordpress, for all the other visitor it's not possible to autofill the email address
and there is no option to disable this option.
Verify the config of your user to see if is the email registered in your WP