已成为共享主机的MX Entry问题。
我必须设置MX条目将邮件发送到外部主机而不是内部邮件服务器（没有意识到邮件服务器是外部的）。

Ended up being an MX Entry problem with the shared host. I had to set the MX Entry to send mail to an external host instead of the internal mail servers (it didn't realize the mail server was external).

问题解决了，一切都很好。

Problem solved and all is well.

编辑：现在我们遇到了这个问题，多次使用主机。
本质上，cPanel给您一个选项，可以说邮件服务器是外部的，但是实际上在进行更改时往往无法实现。我们必须删除邮件服务器并对外部硬编码，以使其正常工作。

We have run into this issue multiple times now with the hosting. Essentially, cPanel gives you an option to say that mail servers are external, but it often fails to deliver on actually making the change. We have to remove the mail servers and hardcode the external one in order to make it work properly and consistently.