发送给客户的邮件总是退回

我发邮件给其中一个客户，以前邮件来往都很正常，但10月份开始，发送过去的邮件总是被退回，是什么原因呢？退回的邮件信息如下：

您发送的邮件被退回，相关信息如下：
    发件人：XXXX
    主题：Re: Re: Unique model electric pencil sharpener.You can always get the right point.
    原文：邮件原文包含在附件中。
    时间：Mon, 12 Oct 2015 10:40:06 +0800
无法发送到 YYYY
    系统应答：(2)the mta server of collarsandcuffs.com -- mail.collarsandcuffs.com(64.34.159.65) reply:571 Delivery not authorized, message refused at 2015-10-12 10:40:11
    退信原因：在向对方服务器发送时出现未知错误。
    解决建议：联系管理员解决或重新发送。

Sorry to bounce your email, Information related below:
    Sender: XXXX
    Subject: Re: Re: Unique model electric pencil sharpener.You can always get the right point.
    Original: Original email has been attached.
    Date: Mon, 12 Oct 2015 10:40:06 +0800
Can not deliver to YYYY
    Reply:(2)the mta server of collarsandcuffs.com -- mail.collarsandcuffs.com(64.34.159.65) reply:571 Delivery not authorized, message refused at 2015-10-12 10:40:11
    Reason:Unknown error happened while sending email to other side.
    Solution:Contact administrator for help or have a try to send again.