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In the upper right corner of the preview pane are buttons for often used actions like reply or forward the message. The rightmost icon will open the message in a new window to have a full-size view on the entire message.
Toggle Message Headers
The preview pane is meant to give you a quick view on the message. Thus only important message headers such as sender and date are displayed right away. The header part of the preview can be expanded with click on the down-arrow at the left of the header.