The Inbox is a storage folder for incoming mail. By default, the Inbox window is divided into two panes. The top pane contains a listing of multiple messages in message view (each message is displayed as a single line of text). Beneath that is the Preview pane, which displays the text of the message. The lower scrollbar on the right side of the window is used to scroll through the selected message. If you have more messages than can be displayed in the upper window, you can use the upper scrollbar to scroll through them.
You can modify the height of the upper and lower windows by dragging the dividing bar that separates them. Move the mouse pointer over the bar. When the pointer turns into a horizontal line with a double-headed vertical arrow, you can drag the boundary up or down to adjust the size of the windows .