As I have said, this is related to SPF, we have a feedback loop sent from hotmail to us anytime there is any issue with email sent to hotmail from our IP ranges. There is little to any spam sent to hotmail from our whole IP range(many thousands of IPs), and the servers themselves are not the issue. The method of sending, PHP mail, etc all plays a factor. The mail needs proper DNS records, proper HEADERS in the email, and proper MIME encoding. All of these are items to check, if you have SPF done, then check mail headers and see that they are correct.