see also: https://www.idropnews.com/news/icloud-custom-domains-now-available-in-public-beta-heres-how-to-set-them-up/166313/ One of the biggest problems seems to be that you won’t be able to use a custom domain email address if it’s ever been used for another Apple ID. If your current Apple ID uses your custom email address, you can, of course, set this up with a custom domain, but if you’ve ever used it for another Apple ID — even one that’s been deleted — Apple won’t ever allow it to be used again. According to Apple Support, this is intentional, so it probably won’t change once the feature comes out of beta.

Have exactly the same issue. added an existing domain "domain.com" when presented "only you" or "family", I opted for "family" was presented with entering email adresses for family members, but opted to skip received confirmation-mail from apple with DNS specifications made necessary changes with my domain registrar received confirmation my domain "domain.com" is now linked to icloud+ tried to add an existing email adres: existing @ domain.com -> this failed with same error as above tried to add a new email adres: new @ domain.com -> succesful The strange thing is that emails send to "existing @ domain.com" do arrive in my icloud+ mailbox. However I canot send mail or add this email adres as an alias of my icloud+ email account.