An email header is simply a snippet of code in an email. This snippet of code contains many things about the email you have received. Some of the information the snippet of code contains is the sender and the recipient. More importantly, it contains the email’s route to get to the inbox along with authentication details. Just to clarify, the email header always precedes the email body.
Step One
To get an email header from Gmail, first, open the email within your web browser. Select the drop-down arrow next to the reply button in the top right corner and choose "Show Original".
Step Two
Once the new page opens (example not shown for security reasons), select Ctrl+A then Ctrl+C to copy all of the data within the window on a PC. On a Mac, select Cmd+A then Cmd+C to copy all data within the window.
Step Three
Once all data is copied, head over to Email Header Analyzer Tool and paste (Ctrl+v for PC or Cmd+v for Mac) that data into the tool window. Hit Analyze to get your results.