iCloud custom Domain - Cannot add one person's email

I have setup three custom domains in my Family iCloud+ 2TB account. There are three people in the family. I can setup emails for me and my son across the three domains, but for my wife I can only setup emails for two of the domains.

I was on the phone for a long time trying to sort this out today, the support member had me logout of my wife's iCloud account so that they could run a tool, but I still cannot add this particular email that we've been using for over a decade.

The error I get during domain setup is:

'The verification email could not be delivered to name@domain.com'

After the domain is setup without this one email. My wife can try to add the email address through her iCloud account, where she can see the custom domain. On trying this, there is a new error message:

Error

'There was a problem with adding this email address. Please try again later.'

I don't know how to get around this issue. I can send emails to this email address without issue.