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Go to https://www.gmail.com/ in your computer's web browser. This will open your Gmail inbox if you're logged in. If you aren't logged in, you'll need to enter your email address and password before you can proceed. Click the email that you want to trace to open it. If the email contains multiple responses, click the tab that contains the email which you want to trace before proceeding. It's in the upper-right corner of the email's page. A drop-down menu will appear. You'll find this in the drop-down menu. This will open the email's raw text form, which includes the email header. It's a blue button in the upper-right side of the page. Doing so copies the entire email to your clipboard; at this point, you can proceed with determining the email's sender.
Open Gmail. Select an email. Click ⋮. Click Show original. Click Copy to clipboard.