You can set your domain to give a free email adress via
www.everyone.net and there thousands of domains they are serving at the moment.
In this respect, I think it's a waste of time to try to exlude free email providers from registering your board.
Say, my domain is turkhukuksitesi.com and my other domain turkhukuksitesi.net gives free email via everyone.net's server. Can you guess this domain is a free provider if I didnt tell you or can you think of any list that displays my domain as a "free email provider". I guess not..
But if you are determined to include them, I would suggest regarding services like everyone.net either, not only yahoo or hotmail..
Just my 0.2 cents..