An Administrator on a Microsoft server with the proper permission level can open the mail directory on the server and see all of the user directories inside. Then simply open the directory and see all of the email message files. Since they are plain text they can be opened with a text editor such as WordPad and easily read.
Linux is the same.
I am not sure if XOBNI crawls the user mailbox on the email server. More likely, it is getting the information it needs from the Outlook .pst file that is on the user’s client machine. The .pst file is a little database just for Outlook to use.
I went to a Microsoft Office developers brunch a while back (good food) and I think I remember they said that they were opening a lot of their Office products to developers so they could make apps to tie in to them.
Here is link to the MSN Developers Network that might help you: http://msdn.microsoft.com/en-us/default.aspx