<<< 550 DY-001 (SNT0-MC1-F3) Unfortunately, messages from 188.8.131.52 weren't sent. Please contact your Internet service provider. You can tell them that Hotmail does not relay dynamically-assigned IP ranges. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
It seems as if you're sending from a dynamic IP address. To fix this, you can set up relaying: http://www.howtoforge.com/postfix_re...her_mailserver