回复邮件被退回

您发送的邮件被退回，相关信息如下：
    发件人：**
    主题：Re:**
    原文：原文较大，退信未包含原文。
    时间：Wed, 18 Nov 2015 14:21:17 +0800

无法发送到**@aol.com
    系统应答：(2)the mta server of aol.com -- mailin-01.mx.aol.com(64.12.91.195) reply:521 5.2.1 : AOL will not accept delivery of this message. at 2015-11-18 14:21:31
    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: **
    Subject:
    Original: Original email has not been attached due to its large size.
    Date: Wed, 18 Nov 2015 14:21:17 +0800

Can not deliver to **
    Reply:(2)the mta server of aol.com -- mailin-01.mx.aol.com(64.12.91.195) reply:521 5.2.1 : AOL will not accept delivery of this message. at 2015-11-18 14:21:31
    Reason:Unknown error happened while sending email to other side.
    Solution:Contact administrator for help or have a try to send again.





这个是怎么回事啊？请帮忙解决一下！