Mail View pane is the center pane of Outlook 2010 main window, which shows all of your email items. For adding/deleting columns in mail view pane, navigate to View tab and from Arrangement group, click Add Columns.
It will bring up Show Columns dialog, Select an available column from left side and click Add –> to add a new column. For removing column, select an existing column from left side and click <- Remove to remove it. Once done, click OK to see changes.
Here you can see the added columns in Mail view window.