Hello white-buffalo-09,
Welcome to Apple Support Communities!
We want to help with Custom Email Domain with iCloud Mail. You can use these steps to add an email:
"Add a custom email domain
To use a custom domain with iCloud Mail, you need:
To add a personalized email address for a custom domain that you use with iCloud Mail:
- That email address can't be used as the email address for a different Apple ID.
- If you want to add a personalized email address that is used for a different Apple ID, you need to change that Apple ID email address.
- If you delete that Apple ID, you still won't be able to add your personalized email address.
Learn how to add a custom domain for iCloud Mail on iCloud.com."
This Apple Support page has the full details: Use Custom Email Domain with iCloud Mail - Apple Support
If the address was already previously used with a custom domain for another Apple ID account, it might not be available to be used again with a different account.
We hope this helps.
Cheers!