Custom email address

Hello shishe,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with adding a custom domain to iCloud Mail. Try the steps here to resolve the issue: Use Custom Email Domain with iCloud Mail - Apple Support

"Add a custom email domain

To use a custom domain with iCloud Mail, you need:

• An Apple ID with two-factor authentication turned on.
• iCloud Mail set up on your devices.

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."

If you still run into issues, reach out to Apple directly for assistance here: Get Support

We hope this helps.

Cheers!