In Gmail you can click the "show details" option to view some of the basic message headers such as who the email was to and from etc, but you can’t see the full raw message headers and message. There is a way to do it that isn’t particularly obvious and this post looks at how to do this.
When you view an email message in Gmail there’s a little down arrow button next to the reply button. This is shown in the first screenshot below highlighted by me with a red box.
Click this button and a menu will appear as shown in the screenshot below. As well as some other useful options there’s an option to "show original" which I have again highlighted with a red box.
Clicking the "Show Orginal" option will open a new window which displays the full raw email messasge including all the headers, as shown in the screenshot below. I’ve blurred out email addresses etc in the screenshot and the big blur at the bottom of the image is the actual message part of the email blurred out.
So that’s how to view the full email message including headers etc in Gmail. It’s very easy to do but easy to miss if you’ve never clicked the arrow button.