email being blocked

I am the secretary for our school group. Just today I received this message in my Thunderbird inbox:
Diagnostic-Code: smtp; 550 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. I have about 50 people in this group that I communicate with regularly. I cannot even send to one person individually.
Any suggestions?