Doesn't accept mediacom cable addresses either (mchsi.com). I had to use my work email. It's a good forum, but the owner doesn't seem too technically savvy, he just blocks whole ISP's based on one individual rather than getting more specific as we've had to do with other boards (of course some of that could be the software running the site too, but his explaination leads more toward the technically savvy response).
That address is, from what I have heard, a very unique case. There is one very unscrupulous dealer who tried to break up deals, reneged on deals, flipped guns hourly on the forum, etc., who used that account. After being banned, he kept opening new fake accounts with that address. The only way to stop it was that approach.