If you hover the mouse just below the senders name it will show you what email address it originated from in the same way as the "old" email interface did. When you hover your mouse the pointer will change to a "hand pointing" when you hover over the viewable spot. This will not work if the name is an email address rather than a name.
If you want to view the full source of the email select the email with a signal click the box to the left of the email in the list. This will not open the email and you then go to "More" in the menu bar and then "View Source".
