Thanks for trying to help, let me explain better.
The email validation is simple but the form still performs the registration as I don't know how to cancel the registration.
I've added some sudo code below, I can check the condition but I can't prevent the registration
$email = get_email();
//what do i put here???
add_action('init', 'update_post_vars_for_registration', 1);
If i put an exit in the stop registration condition it won't register but I need a way to prevent resitration and return to the form and display a message.
hope that makes sense?