MSN Postmaster
As we all know, there is much spam flowing throughout the Internet. Hotmail (amongst others) is a favorite location to send mail from or "spoof" for many spammers. MSN understands this and is fighting back. They have created a website dedicated to work with other large mail carriers out there and even provide some tools to allow ISP/ESP's to view mail straffic from hotmail to their registered IP's.
If you're a hoster providing email services (e.g. Hosted Exchange), you should really take a look.