Custom domain at icloud+ - User already associated with email
I have added my domain to iCloud+. One of the emails, I have transferred to another person ID. Since there was an MX error, I have removed the domain in order to setup it all over again. When I did, the error message is the user already uses that email. How do I make another person to use the domain?
Thank you for your help.
iPhone 8 Plus, iOS 16