Ximian Evolution is a mail client/everything else Outlook does thing for Linux and other Unix platforms.
See 
http://www.ximian.com/evolution/.
Now updated to support GTK 2.0. Configuration is stored in GConf. You can buy a connector from Ximian that will let you talk to Exchange servers too. (Ask GreigMcGill about it in October.)
Evolution is also a theory CharlesDarwin? came up with.
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