Sendcloud的x_smtpapi具体如何定义？

正在用搜狐的Sendcloud发邮件，用x_smtpapi定义收件人。文档严重不足……和客服试了几次都不行。代码：

    $recipient = "'123456@qq.com', '789011@qq.com'";
    $title = $this->input->post('title');
    $body = $this->input->post('body');
    $x_smtpapi = json_encode(array(
        "to" => $recipient));

    $data=
    array(
    'api_user'=>'postmaster@conglinwang.sendcloud.org',
    'api_key' =>'XXXX',
    'from'=>'xxxx',
    'fromname'=>'xxxxx',
    'subject'=>$title,
    'html'=>$body,
    'x_smtpapi'=>$x_smtpapi
    );

报错为：{"message":"error","errors":["Parameter x-smtpapi is not a valid json stringjava.lang.String cannot be cast to net.sf.json.JSONArray"]}

大牛们看看。谢了！