Ok, tried to add 2 custom domains: One is just for my private use. Added the domain, added an email address, just below my iCloud identity icon, all went fine without a glitch. Changed DNS settings at the domain registrar, completed the procedure. Email service works just fine. Even managed to add it into the stock MacOS Mail app via SMTP account (it duplicates the main iCloud mail account, with all the messages and folders structure, but nevertheless still uses MyCustomEmail @ MyCustomDomain. com address). Another domain I tried to add was for family use. Added the domain. Then it offered to add an email under my iCloud identity icon. It didn't offer to add an email for any other family member. But ok, tried to add one for myself, for now, like for a private-use domain. When confirming the addition, it returned: "APIError.EmailAdd.InUse'. I tried to add 3 different names, all been in use before with the previous email hosting provider, same error: "APIError.EmailAdd.InUse". Then tried to add an email never used before, and it went through just fine. But I need to use the ones I used before, so I deleted the domain and tried to add it as for my private use. But the problem with adding emails remained. Just my 10 cents for Apple to fix it sooner.