the To address should use a session variable. in the Universal email wizard double check the variable name being used for the to address, then click the lightning bolt next to trigger and select that variable.
you may also want to add code to specifically unset that variable at the bottom of the page.
for example if the to address variable is $_SESSION['email'] add this case at the bottom of the page:
<?php if(isset($_SESSION['email'])) unset($_SESSION['email']); ?>