public function doubleOptInContact($databaseID, $email) {
        $data["Envelope"] = array(
            "Body" => array(
                "DoubleOptInRecipient" => array(
                    "LIST_ID"         => $databaseID,
                    "COLUMN"          => array(
                        array(
                            'NAME'  => 'EMAIL',
                            'VALUE' => $email,
                        ),
                    ),
                ),
            ),
        );

        $response = $this->_request($data);
        $result = $response["Envelope"]["Body"]["RESULT"];
        if ($this->_isSuccess($result)) {
            if (isset($result['RecipientId']))
                return $result['RecipientId'];
            else {
                throw new \Exception('Recipient added but no recipient ID was returned from the server.');
            }
        }

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