add #ajax property to email field in node add form usion hook_form_alter I've tried to add the property "#ajax" to email field but It dosen't work, 
I've tried all this :

$form["field_user_email"]['#ajax'] = array(
      'callback' =&gt; 'user_ajax_check',
      'wrapper' =&gt; 'usercheck',
      'effect' =&gt; 'fade',
  );

$form["field_user_email"]['und']['#ajax'] = array(
      'callback' =&gt; 'user_ajax_check',
      'wrapper' =&gt; 'usercheck',
      'effect' =&gt; 'fade',
  );
$form["field_user_email"]['und'][0]['#ajax'] = array(
      'callback' =&gt; 'user_ajax_check',
      'wrapper' =&gt; 'usercheck',
      'effect' =&gt; 'fade',
  );
If the field is provided by the Email Field module it'll probably be:

$form["field_user_email"]['und'][0]['email']['#ajax'] = array(...


If it's just a standard text field it will be:

$form["field_user_email"]['und'][0]['value']['#ajax'] = array(...