public function vrfy($string)
    {
        /* Note: 251 is also a valid response code */
        if (PEAR::isError($error = $this->put('VRFY', $string))) {
            return $error;
        }
        if (PEAR::isError($error = $this->parseResponse(array(250, 252)))) {
            return $error;
        }

        return true;
    }