我邮箱收到一封系统退信，显示无法连接到亚马逊的中继服务器：
This is the mail system at host mail.nigzu.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<xxx@xxx.com>: lost connection with
email-smtp.ap-southeast-1.amazonaws.com[52.76.17.168] while receiving the
initial server greeting
......
我以为是服务器被电信拉黑，或者登录凭证过期：
发现都不是，根据官方文章（解决 Amazon SES 的 SMTP 连接或超时问题）提供的排查步骤，唯独在我的服务器上测试不正常，其他网络环境正常。
后来一想，会不会反而是代理的问题？于是把Amazon的SMTP终结点列入强制不走代理的域名白名单，果然恢复正常了，不知什么时候，中继服务默认走了代理。