Open the Mail app and go to Settings > Reading:
Go to Caret Browsing and toggle the switch below Use the caret to navigate the reading pane to On:
Now that caret browsing is enabled, the Mail app will act like word processor. A text selection cursor will now appear when you click in the body of an email message in the Reading Pane. You can move back and forth in the message using the keyboard arrow keys, highlight text using a combination of the [Shift] + arrow keys, or open hyperlinks nested in the body of the message by placing your cursor next a link and pressing the Enter key.