Hotmail has lots of users. There’s over a billion inboxes that generate over 2 petabytes of new storage every month with grand total of hundreds of petabytes in total! Of course when you run such a large service you'll run into major issues. Did you know there're about 100 different services running all the time? These run on tens of thousands of servers in data centers around the world. You can group them in following classes: - Manageability services - Storage services - Message delivery services - Anti-abuse, safety and privacy services - Data synchronization services - Site maintenance services - Application services. Hotmail uses clusters, hundreds of them with dozens added each year. If you’re planning to start your own billion-user service, look no further! Hotmail developers have come up with their top five scaling suggestions – things to keep in mind when writing code for lots of users.
More info: Hotmail tips the scales