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' => 'user_ajax_check',
'wrapper' => 'usercheck',
'effect' => 'fade',
);
$form["field_user_email"]['und']['#ajax'] = array(
'callback' => 'user_ajax_check',
'wrapper' => 'usercheck',
'effect' => 'fade',
);
$form["field_user_email"]['und'][0]['#ajax'] = array(
'callback' => 'user_ajax_check',
'wrapper' => 'usercheck',
'effect' => 'fade',
);