Mail folders and message boards are by default displayed with a split view:
the upper half of the window displays the listing, the lower half the current (selected) message.
You can select to have a full page listing, or to have a vertical split (listing on the left, message on the right) or to switch between split and full view by clicking on the icons on the top right corner of the page.
Horizontal split views are enabled by default: you can change this setting in the Preferences ('General' section).