I'm thinking maybe it's a problem at hotmail or they for some reason have your server identified as a source of spam. I don't know a lot about that stuff, but maybe you could use some site like this to see if you show up:
http://whatismyipaddress.com/blacklist-check (obviously you'd want to use the ip of your server or wherever your email comes from).
Edit: also maybe something on this page will help:
http://mail.live.com/mail/troubleshooting.aspx