From the Outlook Postmaster...

All Exchange Servers are again accessibly to all Outlook email users. Any email sent to users on affected servers is in queue for delivery to them.

(previous message ...) 

We are currently experiencing an unexpected downtime on several Outlook Exchange Servers. The email users on this server are not able to access their Outlook email accounts at this time. Email you may send to users affected will queue for delivery when these Outlook Servers are processing email again. We will send future updates within 30 minutes.