usually in the transit headers of an email, you can determine the originating IP address.
In outlook 2003, if you open the email, then click view, options, you can view the embedded headers from there.
if you look down the list of information, you'll see the router route that the email took to get to your pop server. I'm pretty sure the last "Received:" in the list furthest down, but prior to the date, is the orginating IP address of the sender.
if you take this IP address and use an IP lookup service like IPWHOIS, you can at least determine the country and ISP of the original sender - assuming that they have not taken any obfusticating precautions in the sending of the email such as the use of a proxy.
if a crime has been comitted, the IP can be used by the police to request the user details of whoever was using that IP at the time of the email being sent etc, but if a proxy has been used then it starts to get almost impossible to trace