There was a case study written a few years back about Hotmail’s migration from FreeBSD to Windows 2000 that provides a facinating insight in to Hotmail as a service as well as how Hotmail works today. The numbers are a bit dated in the document, but still the right scale (over half a million chanages a day between user adds and deletes, thousands of front end servers) as is the description of our front end architecture (web servers, DAV servers, login servers, etc). A lot of the document is also really interesting as it talks about some of the performance and productivity gains that were seen during the move to Windows 2000 (like the private heap that prevents memory leeks, the Perl legacy of the code base (which is why the front end code base has it’s own STR class), the use of Unix tools, like grep/syslog/etc, on a front door and more). Neat stuff that’s a great historical perspective of the Hotmail front end.