Microsoft.Office.Interop.Outlook.Application app = null;
Microsoft.Office.Interop.Outlook._NameSpace ns = null;

private void button1_Click(object sender, EventArgs e)
{
    app = new Microsoft.Office.Interop.Outlook.Application();
    ns = app.GetNamespace("MAPI");
    MessageBox.Show(countRootFolders().ToString());
}

public int countRootFolders()
{
    Microsoft.Office.Interop.Outlook.MAPIFolder rootFolder = this.ns.Session.DefaultStore.GetRootFolder();
    int rootCount = rootFolder.Folders.Count;

    foreach (Microsoft.Office.Interop.Outlook.MAPIFolder subfolder in rootFolder.Folders)
    {
        rootCount += subfolder.Folders.Count;
    }

    return rootCount;
}

public int countRootFolders(Microsoft.Office.Interop.Outlook.MAPIFolder aFolder)
{
    int rootCount = aFolder.Folders.Count;

    foreach (Microsoft.Office.Interop.Outlook.MAPIFolder subfolder in aFolder.Folders)
    {
        rootCount += countRootFolders( subFolder );
    }

    return rootCount;
}