EDIT: OK, just checked the Apple discussion boards, and it seems the server is down. There was a work around that I tried successfully. If you're having trouble sending Email from your mobileme account, try changing the port fro 25 to 587. I did, and it worked.
Doesn't sound like the "server is down" it sounds to me like your provider is blocking port 25. This a common problem caused by ISP's trying to block spam generation from their servers. Switching to 587 is a normal and well documented (in the help file) solution.
Glad you are back in business.