How do I preview emails in SharePoint?

Select one or more emails and click the "View email(s)" button in the ribbon or select the option "View email(s)" the context menu of one of the selected emails. This opens a dialog window with the email preview.

Use the "previous" or "next" buttons (or the left arrow / right arrow key on your keyboard) to view adjacent emails. It is also possible to enter a number to jump to that specific email. 

To view all emails simply select the "View email(s)" button in the ribbon. 
NB: the tool uses the column filtering values and sort order to select the same set of emails.