Hotmail mail admin

I'm looking for a hotmail mail admin or someone with the information I'm looking for. I have a client that is trying to block the world, but only allow certain ip addresses through. It looks like hotmail uses a large pool of ip addresses for attachments so we've had a hard time determining what ip addresses to allow. My client specifically is requesting this be allowed. Does anyone know what addresses hotmail users for their attachment servers or would a hotmail admin be willing to contact me off list with this information? Thanks!