There's a new feedback loop in town, according to Chris Wheeler over on Deliverability.com. It's being offered by Tucows, who is, among another things, a wholesaler of domain names and other Internet services to ISPs and web hosting companies worldwide. They do not sell domain names directly to consumers, but operate the OpenSRS reseller service that is widely and popularly used.
My understanding here is that anybody with domains registered via OpenSRS (through a reseller that uses OpenSRS on the backend), and who chooses to uses their HostedEmail.com email service, is now participating in this feedback loop.
What's a feedback loop? For the un-initiated, feedback loops are mechanisms that allow end recipients to easily complain about mail. When you log into AOL and see the "this is spam" button, and you hit that button, you're sending a note both to AOL, and possibly sending a complaint to the original sender, or ISP of the sender. The mechanism that sends that report back to the original sender (or their ISP) is called a feedback loop. It's a valuable thing that just about every ISP or email sending entity wants access to. If you're an ISP or ESP with lots of clients, feedback loop data helps you figure out which ones generate the most complaints, so you can take action against them. If you're a spammer, you might use that data to suppress people who complain (and take no other action), trying to appear to be a better guy than you actually are. (That's why some ISPs will only offer feedback loops to some folks and not others.)
Spamhaus has a comprehensive list showing most of the ISPs that offer feedback loops. It can be found here.