This FAQ will explain how to set up a auto-responder for your 123-reg mailbox.
Login to you 123-reg control panel.
Click on the Manage Email link.
Select the mailbox you wish to apply the auto-responder to and click on the More button, then click on Auto Responder.
Select the On option and then fill in the message you wish your senders to receive.
Now click on the Save & Exit button.
That's it, to turn the auto-responder off just simply follow the same steps but select the Off option.