[rreichel03]

I jumped onto iCloud emails as soon as it became available this past fall and immediately ran into #6. I set up test email accounts in my iCloud account including my wife's just to see if it would work - Evidently that was a mistake because when I removed then attempted to reregister her email under her iCloud account (same "Family") it blocked her with the same opaque error message mentioned. I reached out to Apple support and after going back and forth for 1-2 months, they finally said it was "by design." Because my iCloud account first set up her email address (then subsequently removed it) it was permanently tied to my iCloud account even though we were in the same family. The support rep said there was nothing more that could be done other than moving to another email provider.

tl;dr; Emails can only ever be registered with one iCloud account ever, even in the same Apple family