>1. Open Outlook.
>2. Double-click the message to open it in its own window.
>3. Click the File tab and select Properties.
>4. The properties for the selected message display in a dialog box. ...
>5. Right click inside the Internet headers pane and click Select All. ...
>6. Close the Properties box.
>
>https://support.office.com/en-us/article/view-internet-message-headers-cd039382-dc6e-4264-ac74-c048563d212c

I kind of remember that was the only way, and that's when I discovered that these are not complete headers - specially after I found a couple of messages I sent to groups where I was member twice, once at work (o'look) and once more at home (tBird). The headers in o'look were much shorter, content reformatted in few cases etc.

BTW, the (5) made me wonder, so I checked the link and ah, of course: "and paste it into Notepad or Word to see the entire header at once." ... because. as usual, M$ develops something on 1024x768 and 20 years later the dialong is the same number of pixels as it was then - never make a resizable dialog or you get exiled from Redmond! Or if it's resized, don't save settings! So yup, one of those (still small today: environment variables, paths in installers, font picker, color picker...).