最近 Gmail 邮箱经常收到这样的邮件：Delivery Status Notification (Failure)，内容和QQ邮箱有关，具体内容大体如下：

Delivery to the following recipient failed permanently:

***@vip.qq.com

Technical details of permanent failure:

Google tried to deliver your message, but it was rejected by the server for the recipient domain vip.qq.com by mx3.qq.com. [240e:ff:f040:28::f].

The error that the other server returned was:

550 Mail content denied. http://service.mail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000726

查了下QQ邮箱自助查询的”收信查询”，发现来自Gmail的邮件被当做垃圾邮件处理了。我的QQ邮件是绑定Gmail自动收取Gmail邮件的，于是将被拒收的Gmail地址添加到白名单。