This can be can be tricky depending on the ISPs and e-mail servers involved along the way, but the place to start is to look at the full e-mail headers. In Outlook 2003, right-click the message, select Options and the header info is in the Internet headers box. The information of interest is contained in the Received: field and sometimes the Message-ID: field. These will usually point to the originating e-mail server hostname.
You can then perform a lookup using various free DNS tools on the Internet - my favorites are dnsstuff.com and samspade.org. Simply plug in the hostname you found in the e-mail header into the DNS record lookup and see what it returns. Depending on the tool you use, you may only get an IP address - others may return detailed information about the ISP, domain name, etc. If all you have is an IP address, you can enter that into ARIN's WHOIS query page and find more information. Alternatively, you can use a neat commercial product called VisualRoute. It will do all the lookup stuff for you and map out the location of the originating e-mail server or ISP graphically.
This was first published in July 2005