The RestrictionZone is generally not a dynamic property, as there is no way for us to be alerted when it changes its size. Usually restriction zones should not be changed or at least they should have static dimensions set in pixels. This means that not all scenarios are possible with it and from what I can gather from your description - this is one of them.
If you want to achieve similar behavior to MS Outlook I suggest you use the RadSplitter and place your content in its panes. You may also find interesting this sample application that mimics the desktop application: http://demos.telerik.com/aspnet-ajax/webmail/
the Telerik team
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