This article applies to 123-reg Servers running Plesk.
This article explains how you can fix the error This address no longer accepts mail on your 123-reg Server.
Generally this error happens when mail for the domain was hosted on the server at some point, but has since been moved to an external mail server. Qmail will still try to deliver the message locally and fail with this error:
To correct this check the following two files for the domain in question:
Remove any lines with the domain which is causing the error from both files and then restart Qmail: