Can't add custom domain email address

I am trying to add a domain that I own. Error says: 'This email is already in use with another Apple ID.' I have already deleted that Apple ID account (I couldn't just change the email associated with it, and went through the process of having apple remove the account, took 2 weeks).

I've tried using the email to log into iCloud.com, to see what Apple ID was using that email, but it says "this email is not in use"...

I have set up emails with 3 other custom domains with iCloud, and had no issues. This one is just not working.

Any help would be appreciated!