On the Space Shuttle and the International Space Station they use Microsoft Outlook 2003, but not quite in the same way that us earthbound Earthlings do. The space shuttle Atlantis is orbiting the earth right now and the crew exchange emails with the ground a few times each day. Bandwidth is a constraint and you don’t want the busy crewmembers bothered with spam or unnecessary messages so NASA has a special system in place.
The crew use fairly standard laptops running Microsoft Outlook (currently Outlook 2003) with Exchange Server as the email host, but they don’t link to the server using any of the standard methods. Instead of exchanging messages one-by-one via a network link, all incoming and outgoing messages are bundled up in a file. That file is moved from ground to orbit and later back again.

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According to a notification from NASA IT department , they are in the process of upgrading the environment to Exchange 2007.

According to Microsoft News Center, the first time astronauts have used Outlook from space was in 1998.

You can download 2010 International Space Station Calendar here.

-Thanks, Jinesh.