To configure your domain hosted on 123 Reg with SPF, you will have to add them to your SPF record. The SPF mechanism used by 123 Reg is shown below.
mx a ip4:22.214.171.124
If you are using 123 email, it should be:
mx a ip4:126.96.36.199 a:mailex.mailcore.me
For more information please click the button below.
You can configure your 123 Reg domain with DKIM via your 123 Reg Control Panel as described here:
Create a free OnDMARC account to test your configuration.