To delete a mailbox, you first need to log into your 123-reg control panel, then follow the instructions below:
Scroll down the page until you reach the Email section.
Click on the Manage email link.
Find the mailbox that you wish to delete by first selecting the relevant domain from the domain drop down menu.
Select Delete Address from the More menu.
You will then be advised that by deleting the mailbox, you will lose all emails currently stored in it. To confirm that you wish the mailbox to be deleted click on the Yes button.