I run the Windows Live business in Europe, the Middle East, and Africa and just learned quite a bit more about our Hotmail datacenters in this blog post by Arthur de Haan. Read the full post for the details, but some highlights are:
We host well over 1.3 billion inboxes (some users have multiple inboxes)
Over 350 million people are actively using Hotmail on a monthly basis (source: comScore, August 2009).
We handle over 3 billion messages a day and filter out over 1 billion spam messages - mail that you never see in your inbox.
We are growing storage at over 2 petabytes a month (a petabyte is ~1 million gigabytes or ~1000 terabytes).
We currently have over 155 petabytes of storage deployed (70% of storage is taken up with attachments, typically photos).
We have one test engineer for every developer
The full article is: A peek behind the scenes at Hotmail