Look in the email headers, and you’ll find the originating IP address in there.
It’ll say somethign like “Received: from 22.214.171.124 by something.yahoo.com”
and/or “X-Originating-IP: [126.96.36.199]”
You can then take that IP address and put into a reverse lookup site/application, and it will give you information of varying worth. You might try This Place
You’ll at least get an ISP, but for very large ISPs, like AOL or Road Runner, that may not be very helpful. Most large ISPs break their DNS hames into regional, so you may see something like cinci.rr.com. Then you know the person is in Cincinnati.
You could then be so devious as to cross reference that IP, or an appropriate range of IPs, with other emails you may have received from people who are suspect. So if you start to think it’s someone you know, and you’ve got other email from that person (even from different email addresses, as long as it was the same computer that sent them), you can feel more confident you’ve narrowed it down.