KB01120806 What is the error [553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)]?

What is error "[553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)]"?

This is a typical error returned by a mail server when it is not prepared to act as a relay. Usually, it is because the mail server has been misconfigured and is published as the designated mail relay for the recipient in the message.

You should contact the mail server administrators to report the error, or try again later.

If you are receiving this message for all outgoing mail, then it is probably being reported by your smart host, and this is probably because you are not authorised to use that smart host. This frequently happens when you have multiple dial-in connections but are using the same smart host for all of them.