Every e-mail that I have sent today to friends with a Hotmail account has been returned saying "550 REPLY: 550 SC-004 Mail rejected by Windows Live Hotmail for policy reasons. A block has been placed against your IP address because we have received complaints concerning mail coming from that IP address. If you are not an email/network admin please contact your ..." or something similar. What do I do?

Seeing as how you had spam links in your signature, you were probably reported as a spammer from said account. I'd say you deserved it. Bye bye, now! -P.