whmcsZimbraSingle/zimbraAddressAvailable.php

<?php
$pos = strpos($_SERVER['HTTP_REFERER'],getenv('HTTP_HOST'));
if($pos===false) {
    die('Restricted access');
}

require_once(__DIR__ . '/../../../init.php');
require_once("api/Zm/Auth.php");
require_once("api/Zm/Account.php");
use WHMCS\Database\Capsule;

$whmcs = App::self();
$account_name = $_GET['name'] . "@" .  $_GET['domain'];
$productID = $_GET['pid'];
$accessData = array('zimbraServer' => '', 'adminUser' => '', 'adminPass' => '');
$serverGroupIDObj = Capsule::table('tblproducts')
	->select('servergroup')
	->where('id', '=', $productID)
	->get();
$serverGroupID = $serverGroupIDObj[0]->servergroup;
$serverIDObj = Capsule::table('tblservergroupsrel')
    ->select('serverid')
    ->where('groupid', '=', $serverGroupID)
    ->get();
$serverID = $serverIDObj[0]->serverid;
$server = Capsule::table('tblservers')
	->select('ipaddress', 'username', 'password')
	->where('id', '=', $serverID)
	->where('active', '=', 1)
	->get();
$accessData['zimbraServer'] = $server[0]->ipaddress;
$accessData['adminUser'] = $server[0]->username;
$adminPassCrypt = $server[0]->password;
$adminPassDecrypt = localAPI('DecryptPassword', array('password2' => $adminPassCrypt));
if ($adminPassDecrypt['result'] == 'success') {
	$accessData['adminPass'] = $adminPassDecrypt['password'];
}
$api = new Zm_Auth($accessData['zimbraServer'], $accessData['adminUser'], $accessData['adminPass'], "admin");
$login = $api->login();
if(is_a($login, "Exception")) {
	logModuleCall(
		'zimbrasingle',
		__FUNCTION__,
		$accessData,
		"Error : cannot login to " . $accessData['zimbraServer'],
		$login->getMessage()
	);
	exit();
} else {
	$apiAccountManager = new Zm_Account($api);
	if( $apiAccountManager->accountExists($account_name)) {
		echo 'no';
	} else {
		echo 'yes';
	}
}