By default, Google Mail shows only a high-level representation of an email’s headers: who it was sent from and to, the date and the subject. If you want to see more information from the headers, you can if you know how.

The trick is to switch back into basic HTML view by clicking the link at the bottom.

Then navigate back to the email you’re interested in, and you’ll find a Show original link that you can click to see the full raw text of the email, including all its headers.