In order to add 123-reg MX records for your domain name so that 123-reg email works, please do the following:
Firstly, you need to ensure your domain is pointed to our nameservers:
If the domain is not pointed to the 123-reg nameservers, you will be unable to manage them from the 123-reg control panel.
For more information on changing your nameservers please view the following article:
How do I change the nameservers for my domain name?
Please Note: It will take between 24 - 48 hours for the new MX records to become active.
In the Domain names section, select the relevant domain name in the drop-down list and click on the Manage button.
Scroll down to the Advanced Domain Settings section and click on the Manage DNS (A, MX, CNAME, TXT) link.
From the Basic DNS tab.
Click on the Change button in the EMAIL SETTINGS (MX RECORDS) section.
Select 123-reg email from the Your email is pointing to: drop down list.
Click on the Point to service button.
Click on the Confirm button to complete the change.