Correction:
I am trying to write a C# application that has a GUI similar to Outlook
Express.
What exactly do you need to "look like Outlook Express"? It would seem
that WinForms provide all the necessary controls for that - menu bar,
tool bars, lists etc; the only thing that's missing is a multi-column
TreeView, but if you look on CodePlex (and elsewhere), you'll find
plenty of open source implementations as well as commercial ones.
I have used a 3rd party tool. C1NavBar which gives outlook 2003 look.