public function unsubscribe($email_id, $delete_member = false, $send_goodbye = true, $send_notify = true, $email_identifier = 'email')
    {

        if (!in_array($email_identifier, array("email", "euid", "leid")))
            throw new InvalidArgumentException('email identifier should be one of ("email","euid","leid")');

        $payload = array(
            'id' => $this->listId,
            'email' => array(
                $email_identifier => $email_id
            ),
            'delete_member' => $delete_member,
            'send_goodbye' => $send_goodbye,
            'send_notify' => $send_notify
        );

        $apiCall = 'lists/unsubscribe';
        $data = $this->requestMonkey($apiCall, $payload);
        $data = json_decode($data, true);
        if (isset($data['error']))
            throw new MailchimpAPIException($data);
        else
            return true;
    }