The form posts to itself and then redirects to the thank you page, so you can't refer to the form post on the thank you page directly.
The solution is to save the email address into the session on the top of the email page so you can retrieve and display it from the session on the thank you page. You can use the webassist "Persist Form" to save the entire submitted form to the session and retrieve it that way, or just use the "Set Session Value" server behavior to set a single session variable from the email field.