Domains
Depending on your Viewpoint configuration you may only allow access (allow list) to certain domain names, alternatively you may have excluded (block list) access certain domain names.

Example:
If ‘alloweddomain.com’ is added to your allow list, all users with email addresses under this domain E.g user@alloweddomain.com will now be allowed to register
If ‘blockeddomain.com’ is added to your block list, all users with email addresses under this domain E.g user@blockeddomain.com will not be able to register