Trying to Set up Custom Domain Email - Keep Receiving ''Check your MX record''

Hi Everyone,

I have a domain from Porkbun, and trying to set up the custom domain email address with iCloud. I followed Apple's instructions correctly and yet after 72 hours I still keep receiving the same message:

Check your MX record

Make sure the settings that you updated on your domain registrar match the ones sent to you. If you just updated the information, try this again later. It can take up to 72 hours for your changes to take effect.

I surfed the forum, but did not find any plausible solution. Does this service of a custom email domain even work?