When you receive an email, you receive more than just the message. The email comes with headers that carry important information that can tell where the email was sent from and possibly who sent it. For that, you would need to find the IP address of the sender. The tutorial below can help you find the IP address of the sender.
Note that this will not work if the sender uses anonymous proxy servers.
Also, note that if you receive an email sent from a Gmail account through the web browser, you may not be able to find the real IP address because Google hides the real IP address of the sender. However, if someone sends you a mail from his/her Gmail account using a client like Thunderbird, Outlook or Apple Mail, you can find the originating IP address.