The RadPanelBar is a hierarchical ItemsControl with multiple selection, that can be made to look like a outlook side bar but certainly it would not match its features.
We are currently working on an Outlook-like PanelBar, here is a screen:
I am sending you a project with a prototype of the control.
Please note that this is untested prototype code. What I would like to do is show that we will soon have an outlook-sidebar-like control for you to use.
All the best,
the Telerik team