public function sendMailing($emailID, $mailingID, $optionalKeys = array()) {
        $data["Envelope"] = array(
            "Body" => array(
                "SendMailing" => array(
                    "MailingId"         => $mailingID,
                    "RecipientEmail"    => $emailID,
                ),
            ),
        );
        foreach ($optionalKeys as $key => $value) {
            $data["Envelope"]["Body"]["SendMailing"][$key] = $value;
        }

        $response = $this->_request($data);
        $result = $response["Envelope"]["Body"]["RESULT"];

        if ($this->_isSuccess($result)) {
            return true;
        } else {
            throw new \Exception("SendEmail Error: ".$this->_getErrorFromResponse($response));
        }
    }