When selecting an email from the list, by clicking on it, its common headers and body will be displayed in the preview pane.
By default, the preview pane displays the sender, recipient(s) and of the email, horizontally. To have them displayed vertically, press the "+" icon in front of the subject line, as shown in the below picture.
In the vertical view, the "To" and "Cc" fields are also displayed.
In the lower part of the preview pane, you can click the "View Source" button to open the email's full source, including headers and body, in a new tab. Click the "Block sender" button to add the sender of the selected message to your account-level blacklist.
To view a message in a separate window, double-click on it in the email list found above the preview pane.
The view message window has the same behavior and options as the preview pane.