That's cool. I just had a look, and noticed a couple of rbl checks. Very smart idea.

However, I have to say, I had occasions where the IP was not listed on any RBL, but Yahoo was still flagging my IP.

I don't remember the exact error Yahoo were returning, but I think it was basically blocking all email... Perhaps even from the entire network range, since it was listed as residential or something of that sort (apologies, but I really don't remember the exact details, it was quite a while ago)