Register.php

               ->add(
                   array(
                        'name'       => 'email',
                        'validators' => array(
                            array(
                                'name'     => 'callback',
                                'priority' => PHP_INT_MAX,
                                'options'  => array(
                                    'callback' => function($fo) {

                                    }
                                )
                            )
                        )
                   )
               );