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.
View your email's full headers
1. From a browser, open Gmail.
2. Open the email you want to check the headers for.
3. Next to Reply, , click on the Down arrow .
4. Click Show original.
5. The headers will show in a new window, including fields like authentication results.
6. To get the full message header, click Download original.