Clarity

Hi
Can you offer some clarity as to how this works?
I presume that you add domains to the whitelist and then they are the only domains that can register ?
I have tried this and it does not seem to work.
But if I add a domain to the blacklist it does work it denies those people.
The case I am dealing with is an ultimate member site where I want to restrict account creation to a know whitelist of domains and not allow any other domains. Can I achieve that with this plug in ?

Thanks for your help.
Dave