* Log into your Yahoo! mail with your username and password.
* Click on Inbox or whichever folder you have stored your mail.
* Open the mail.
* If you do not see the headers above the mail message, your headers are not displayed. To display the headers,
* Click on Options on the top-right corner
* In the Mail Options page, click on General Preferences
* Scroll down to Messages where you have the Headers option
* Make sure that Show all headers on incoming messages is selected
* Click on the Save button
* Go back to the mails and open that mail
* You should see similar headers like above
Or if you want to manually find the IP address, proceed to 6.
* Look for Received: from followed by the IP address between square brackets [ ]. Here, it is 18.104.22.168.
That is be the IP address of the sender.