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Error message "553: Sorry, that domain isn't in my list of allowed rcpthosts"

The reason for the error "553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)" is that your mail application has not authenticated to our mail server.

In order to send mail through our server, your email program must authenticate.

That can be done by entering the full email address of the mailbox and the password in the outgoing SMTP mail server settings of your email application so that it supplies them before sending. This can be configured in the options for your email account in the application.