function get_user_email($user_id = null) {
    $user       = \get_user_by('id', $user_id);
    $metas  = \get_user_meta($user_id);
    $email  = \get_the_author_meta('user_email', $user_id);
    return isset($metas['user_email'])  && !empty($metas['user_email']) ? $metas['user_email'][0] : (!empty($email) ? $email : $user->user_email);
 };