As mentioned in the Hotmail's reply:
Please contact your Internet service provider since part of their network is on our block list.
So you will need to contact your ISP, and ask them to contact hotmail to get the block removed from the Hotmail's block list.
It will/should help if you give your ISP the link that is in the Hotmail's reply: http://mail.live.com/mail/troubleshooting.aspx#errors
, and tell them that it's about SMTP Error code: 550 SC-001
Code 550 SC-001 reads:
Mail rejected by Windows Live Hotmail for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.
Basically spam is beening send from the IP block that you are on.
After this you will need to setup the needed reverse DNS and SPF record for your domain(s)