This article will explain how you can configure reverse DNS/PTR records on your 123-reg VPS.
In the VPS section, click on the Manage link.
Click on the Manage button next to VPS you wish to configure.
Click on the IP tab.
Click on the Edit link for the IP address in the Reverse DNS section you wish to change.
Select the domain from the drop down list you wish to change your IP address to.
Click on the Assign Domain button and then click the Yes button to confirm.