custom domain email "This email address is not available. Please try again with a different address"

Sorry if this has been answered before. I am looking into existing ones but nothing so far seems to be quite this issue.

I moved my custom domain email from Gmail to iCloud+. I set the DNS records and successfully added an email account in iCloud. I removed the Gmail DNS records.

This issue is it won't let set up the exact same email address. Let's say my old email is "foo@bar.com". In iCloud, I can create "foo2@bar.com" but if I try to add "foo@bar.com" it gives me an error box with the message I entered in the subject line.

I even set "Allow all incoming messages" and it works EXCEPT for my actual specific email address.

This is not good. I've used that original email for many years and I really need it working. I don't understand the issue. I own the domain and iCloud has accepted its use.