I have never seen such a picky forum when it comes to registration. Here's a list of the email providers I tried to use and were not accepted.

1. Hotmail (obvious)
2. Yahoo
3. Netscape
4. Gmail

I had to go search the net for some no-name email provider, and finally was accepted I think it should be listed in the help files which providers are blacklisted, or at least have an error message telling you why the registration failed.

Yes, i agree it's not clear enough. But it won't be changed before IPB 2.0 final is out and installed...