This document provides a 4-step process for tracing an email back to its source:
1) Examine the email header, paying close attention to the "Received:" fields which show the path of the email.
2) Work backwards through the "Received:" fields to identify the originating IP address.
3) Use internet registries like ARIN to identify which organization owns that IP address.
4) Contact the organization and ask them to check login records to identify the specific user at the source IP.