Restricting email domains for self-registering Ideas portals

Who would benefit?

B2B Ideas customers

What impact would it make?

Easier maintainability of Idea portal access

How should it work?

As B2B SaaS company working with an extensive partner network within a competitive space, I want to be able to add a list of email domains that I don't want to be able to self-register in a password protected Ideas portal. Example, "public email services" such as google.com + competitor email domains.

Basically the opposite of how the Employee domain field can be tweaked and used today, maintaining a (long) list of approved domains.