Ajax - validate an input field after form submit

Hi,

I'm trying to validate an input field after form submission but the ajax callback seems not to look after my condition. Here is the code:

/**
 * Perform alterations before a form is rendered.
 */
function MY_MODULE_domaincheck_form_alter(&$form, &$form_state, $form_id) {
  
  if ($form_id == 'user_register_form') {
     
    $form['account']['mail'] = array(
      '#type' => 'textfield',
      '#title' => 'E-Mail',
      '#maxlength' => 254,
      '#description' => '',
      '#required' => TRUE,
      '#default_value' => '',
      '#ajax' => array(
        'callback' => 'ajax_email_domain_callback',
        'wrapper' => 'my_wrapper',
        'effect' => 'fade',
        'event' => 'submit',
      ),
      '#prefix' => '<div id="my_wrapper">',
      '#suffix' => '</div>',
    );
    
    $form['actions']['submit'] = array(
      '#type' => 'submit',
      '#value' => 'Registrieren',
      '#ajax' => array(
        'callback' => 'ajax_email_domain_callback',
        'wrapper' => 'my_wrapper',
        'effect' => 'fade',
       ),       
    );
             
    // THIS PART IS NOT WORKING AFTER THE FORM HAS BEEN SUBMITTED
    if (!empty($form_state['values'])) {
      
      if (MY CONDITION) {
        form_set_error('', 'MY ERROR MESSAGE');
      }
    
    }
  
    return $form;  
         
  }
  
}

function ajax_email_domain_callback($form, $form_state) {

  return $form['account']['mail'];
    
}