Most likely either the email host/domain you are using (i.e. hotmail.com, gmail.com, yahoo.com, whatever.com, etc) or your ISP range has been listed by Windows Live Hotmail as spammers or some other blacklist user. The only real solution is to contact Hotmail tech support and inform them that your domain is being rejected.
Here's a link generally discussing the problem. You *may* have a much more specific problem that can be fixed, but based on the info you've given us, this is the most likely cause.
Good luck!
http://www.eggheadcafe.com/software/...id-550-sc.aspx