Particular email address for custom domain says “try again later”

I added a custom domain. I’ve done it before and this is my second domain so it’s not a DNS/NS issue.

I was even able to create two new emails @domain but the one email that was previously setup in Mail and still has emails showing in it, I cannot add. I keep getting the, “We cannot process your request, please try again later”. I’ve waited two weeks and as I said earlier which bears repeating, I was/am able to add other@domain but not this specific one.

Any ideas.

Thanks