whmcsZimbraSingle/zimbraAddressCheck.php

<?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("CLIENTAREA", true);
require_once(__DIR__ . '/../../../init.php');
require_once(ROOTDIR . '/includes/dbfunctions.php');
require_once dirname(__FILE__) . '/zimbraSingle.inc';

$whmcs = App::self();
$whmcs->set_req_var('id', $_GET['pid']);
if(zimbraSingleDoesEMailExist($_GET['name'], $_GET['domain'])) {
	echo 'no';
} else {
	echo 'yes';
}