Nathe Forden I would definitely not recommend blocking those providers. I myself use a Live (Hotmail) account daily. I don't see anything wrong with it and so far I am not having any issues with mail being sent out via my SMTP mail server. I have had a lot of problems before with Postfix on a servers running Ubuntu. I don't like it personally.
Cem Bagdatli I would be interested in seeing how this works out. Also, I would like to talk more about your portal. I myself am working on something as well but it was using a different platform. I would like to see how UNA is used for that particular project.