This is for Windows only.
Add Message Options to the Quick Access Toolbar by clicking the little down arrow in the upper left corner and choosing More Commands...
1. Choose All Commands from the top drop down.
2. Scroll until you find Message Options... then click to select
3. Click Add in the middle
4. Message Options... should now show up as a chosen command
5. Click Ok
You should now see the Message Options icon in your Quick Access Toolbar
Select the email from which you wish to get the headers
Then click the Message Options icon
A box similar to the one below should appear
Copy the data in the Internet headers box by clicking within the box and hitting Ctrl+A to select all text then hit Ctrl+C to copy the highlighted text
Ctrl+V the data into our Email Header Analyzer tool and click Analyze.