public function isDisposable($email)
    {
        if (! $this->isEmail($email)) {
            return null;
        }

        $hostname = $this->hostnameFromEmail($email);

        if ($hostname) {
            // Load disposable domains
            if (is_null($this->disposable)) {
                $data = new \EmailData\Data();
                $file = $data->getPathToDataFile('php');
                $this->disposable = include($file);
            }

            // Search array for hostname
            if (in_array($hostname, $this->disposable)) {
                return true;
            }

            return false;
        }

        return null;
    }