This error occurs when someone is trying to get mail from outside of their ISP's network.
Many ISP's force their users to POP before they SMTP if they are connected to the Internet via a third party. This is done to prevent spam being sent.
They can send mail, but, they must first download it. When they download it they verify themselves!
The error they could get is ;
Sorry, that domain isn't in my list of allowed rcppthosts (#5.7.1)
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