Interesting error, can you provide more details please? Does this happen with ALL email sent to hotmail or just specific email? Are you sending from an exchange account or some other mail system, if so what mail system? If Exchange what version and service pack level? Are you the only one experiencing the issue or are there others having the same problem? Can’t really help you with just the information you provided. Thanks!
Your domain is probably blacklisted. Make sure your server is not setup as an Open Relay, and make sure you are not sending out spam on your domain.
Additionally you can use this link to check if your IP address is on the black list.