public function validateForm(
  array &$form,
  FormStateInterface $form_state
) {
  $value = $form_state->getValue('email');
  if(!\Drupal::service('email.validator')->isValid($value)) {
    $form_state->setErrorByName(
      'email',
      t('The email address %mail is not valid.', ['%mail' => $value])
    );
  }
}