This guide is for newer versions of Outlook
1.Double-click the email message to display it in its own full window.
2.Select File > Properties.
The Properties window is displayed.
3.View the message header in the “Internet headers” field at the bottom of the window
Tip: You can highlight the information in that box, press Ctrl+C to copy, and paste it into Notepad to see the entire header at once
For Web Access;
1.Select the three dots (…) located at the upper-right corner while in the message
2.Choose "View" and select "view message details"
3.The message header is displayed