Add an action to user register form
You can add additional submit callback function to the registration form, then assign user's email to a session so you can call it on the destination page.

Add the following line after $form['#action']

$form['#submit'][] = 'custom_user_register_submit';

Then add this function to your custom module 

function custom_user_register_submit(&amp;$form, &amp;$form_state) {

  $_SESSION['custom_user_mail'] = $form_state['values']['mail'];  
}


Then call $_SESSION['custom_user_mail'] on any page you want.