in yahoo mail at the top of box which displays subject and senders email id, there will be an option which says full headers, diaplay the heders, copy the IP address and then open the site *www.whois.com/(paste ip address and then u r done
To view the time or date a message was sent, or to view the email address of the person who sent you a message, click 'More options' and then click 'Show original.' This displays the full header of a message.