maybe the IP, from which you try to connect to hotmail, is a dialup/home user IP. Hotmail knows about these IP ranges and - AFAIK - blocks them.
Try connecting by hand, maybe that gives you more info:
telnet 188.8.131.52 25
<Look at what the other side says>
<Put in your hostname, e.g. dig -x your-external-ip>:
<see what hotmail says>
Post the full text of that "conversation" here, don't cut away anything.