Weighing in with an opinion, as bad as blacklists *may be*, at least they let the sender know something's up. Not in an artful way, to be sure, but they give some notice. The sender can do _something_, including dropping his association with the recipient b/c it's not worth his time and trouble. Blackholing email because you think it's spam, OTOH, is pure evil.
Host type can only be used as a relatively small weighting factor toward blocking connections. However in the absence of any other reputation data on a particular IP, it's a safe way to trigger throttling or rate limiting. IMHO receivers have a right to filter traffic in any way that reduces abuse while serving the needs of their end users. There is a lot of pressure from end users and legitimate email senders to ensure that whatever blocking strategy is in use ensures that the good stuff is not blocked. Regards, Ken -- MailChannels: Reliable Email Delivery (TM) | http://mailchannels.com -- Suite 203, 910 Richards St. Vancouver, BC, V6B 3C1, Canada Direct: +1-604-729-1741