Why am I getting this error message when I try to add a new domain? "Domain should be unique across inbound and white listed sender domains"

Go to Block/Accept ip filters, recipient filters and sender filters and check for and remove any entries which reference the domain they are trying to add.

The domain they are trying to add cannot be white listed in any of these places. Once the domain is deleted from any white lists go to Domains, Domain manager and add the domain.