Inside the message headers you should see some "gibberish" like this (using a PP forum email since it doesn't contain any private info we don't already know):
Received: from [207.136.153.227] ([207.136.153.227:3883] helo=FORUMS.PELICANPARTS.COM)
by cdptpa-iedge02.mail.rr.com (envelope-from
)
(ecelerity 2.2.2.39 r()) with ESMTP
id DF/87-20510-8A9562C4; Sat, 26 Jun 2010 19:48:56 +0000
The IP address is the 207.136.153.227 portion.
Then you can plug it into an NSLOOKUP and/or WHOIS query such as DNS tools.
DNS Tools!
This tells you what domain name and server the IP address maps to, if it is registered with DNS (domain name service). It also tells you the ISP providing the IP address, but that is not terribly useful in itself.
In this case we see the domain name is pelicanparts.com so we can do a WHOIS lookup on that domain name:
DNS Tools!
Then you get the registered name and address of the domain owner, or his/her designated representative in the case of private registrations.
Hope that helps.