Cloudflare and apple mail

I went through the process of adding a custom domain to my apple mail account. I switched my domain registrar to Cloudflare (cheaper, etc). But my custom email addresses (just 2 of them) still do not work. All messages, sending and receiving, bounce. In my mail accounts I see that my host name reflects my old registrar. What should my new host name be? Are there other settings that need to be changed? All I want to do is use my registered domain name to send and receive via my iCloud mail account.

This shouldn't be that hard. (All of these questions apply to my iPhone and iPad as well.)

Thanks for your help,

Jim H