spekesoftly may be heading in the right direction. My organisation had an issue with hotmail and some other websites (all based in the US), turned out to be a peering issue for our ISP. May be worth contacting your ISP and ask them to check their peering routes for hotmail.
For those of you who want to know why ISP peering is important the following links are good guides
ISP Peering &
another good site