When attempting to send a email using your 123-reg Server, your email client may report the following error message:
"553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)"
However, you are able to receive email without issue.
This error message is reported by your email client because the check box labeled "My Server Requires Authentication" is not checked. Your email client attempts to send the email to the server without providing a password, hence the server replies that you are not allowed to relay without proper authentication.
To resolve this issue you will simply need to check the checkbox next to "My Server Requires Authentication" in your email clients settings.