How do I trace an email with its full headers?
For an email you received in Gmail, you can see where the email came from by looking at its headers, including how it got from the sender to the recipient's mail servers.
From a browser, open Gmail.
Open the email you want to check the headers for.
Next to Reply, click More, Show original.
The headers will show in a new window, including fields like authentication results.
If you need to provide the full headers of an email for troubleshooting:
Select all the text in the new window, right click and select copy, and paste in a new text file. (Do NOT select "Download Original")
To learn more, check out Google Support.