I may as well pick this functionality to get started on developing for XF. I haven't really coded in PHP but I know enough C++ and C# to get exceptions thrown. What I would like to do is implement a system of restrictions to the email addresses people can use to register. what is the permission order? allow/deny or deny/allow what is the default setting? allow from all or deny from all allow list: specific.com some.net another.org deny list: gmail.com hotmail.com yahoo.com logic to make this all happen and display a descriptive error message if user can't register The thing is, I can probably code the logic in PHP just fine, I just don't even know where to start looking how to integrate it into XF. I do know that the options would need to go into the User Registration AdminCP page. Is there some soft of chart or ERD that shows what template is part of what and which hook is where etc?