Open the specific email message of interest in a new window by double-clicking its line in the email summary pane.
Go to the “File” tab in the new window and click on the “Properties” button.
Note that if you go to the “File” tab in your main Outlook window (as opposed to the window that you popped up with a single email message), it will not have a “Properties” button, because these are specific to a single message rather than the whole application.
When you click the “Properties” button, a new Properties window should pop up:
The headers are in the bottom portion of the window. Copy and paste to an editor in order to view them in a more convenient manner.