By default, Windows 10’s Mail app displays the first line of each email in addition to the subject line. If you don’t want this preview text to show, you can easily turn it off.
Maybe you work in public places, or maybe you have nosy family and friends. Whatever the reason, if you don’t want others seeing the content of your emails, we’ll show you how to hide the preview text. Mail also has a preview pane that you can hide as well.
To disable the message preview text, click the “Settings” icon in the lower-left corner of the Mail window.