ReturnPath has announced that they are aquiring BondedSender from IronPort Systems.

BondedSender is a dns whitelist that companies must pay a bond ($$), and go through a TRUSTe privacy certification in order to get listed on. It is however free for anyone to use the bonded sender whitelist, and Hotmail, and RoadRunner are the biggest users of the bonded sender whitelists for filtering. When your on this list you can skip lots of spam filters.

ReturnPath's CEO Matt Blumberg explains BondedSender as follows on his blog:

Bonded Sender is the industry's oldest, best known, and most effective whitelist/accreditation program. In a nutshell, it's a bitch for mailers to qualify for it -- they have to demonstrate that they're a super high quality mailer and get certified by our partner TrustE -- but once they do, they have relatively guaranteed safe passage and default images into the inbox at Microsoft (Hotmail and MSN), Roadrunner, and a number of smaller ISPs plus over 35,000 corporate domains who use SpamAssassin or who have Ironport's email appliances installed at their gateway.
BUT -- and this is a big but -- they have to keep clean in order to stay on the list, and if they receive more than a tiny number of spam complaints against them, they get fined (hence, the Bond) and ultimately kicked out of the program.