public function optOutContact($databaseID, $email, $columns = array()) {
        $data["Envelope"] = array(
            "Body" => array(
                "OptOutRecipient" => array(
                    "LIST_ID"         => $databaseID,
                    "EMAIL"           => $email,
                    "COLUMN" => array(),
                ),
            ),
        );
        $columns['EMAIL'] = $email;
        foreach ($columns as $name => $value) {
            $data["Envelope"]["Body"]["OptOutRecipient"]["COLUMN"][] = array("NAME" => $name, "VALUE" => $value);
        }

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

        if ($this->_isSuccess($result)) {
            return true;
        }

        throw new \Exception("OptOutRecipient Error: ".$this->_getErrorFromResponse($response));
    }