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");
}
require_once("api/Zm/Auth.php");
require_once("api/Zm/Account.php");

$account_name = $_GET['name'] . "@" .  $_GET['domain'];
$api = new Zm_Auth('192.168.16.225', 'admin@thurdata.local', 'Technics2312', "admin");
$login = $api->login();
$apiAccountManager = new Zm_Account($api);
if( $apiAccountManager->accountExists($account_name)) {
	echo 'no';
} else {
	echo 'yes';
}