I can only presume that thay have had problems with people registering what is quite an anonymous email (like hotmail) and be abusive. I think you need a reasonably permanant email address.
Not bashing them, just annoying that they won't accept web email addresses, which is all I have at home. It's the first forum I have ever come across that won't accept a Hotmail address. Daft considering they have security on their registration page.