it's a server error. So it seems appropriate to contact the system manager or helpdesk of hotmail. I assume that anybody offering a service does have some form of customer support in case of problems. If not, consider switching to another email-provider.

Of course, it could have been a temporary problem. Tried again?

And to be certain of the location of the problem source: try it on another machine. That's easy with web-based services.

Kees