try some access restrictions

zimbraAddressAvailable.php

@@ -1,8 +1,11 @@
 <?php
-$allowed_host = 'admin.seecure.ch';
-$host = parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST);
-if(substr($host, 0 - strlen($allowed_host)) != $allowed_host) {
-  die("This file cannot be accessed directly");
+define('IS_AJAX', isset($_SERVER['HTTP_X_REQUESTED_WITH']) &&      strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');
+if(!IS_AJAX) {
+    die('Restricted access');
+}
+$pos = strpos($_SERVER['HTTP_REFERER'],getenv('HTTP_HOST'));
+if($pos===false) {
+    die('Restricted access');
 }
 
 require_once(__DIR__ . '/../../../init.php');