Suppose a user receives an E-Mail with a URL and follows the link - the target server receives a Referer String containing the PHPSESSION-Id for example: http://e19.email.excite.com/msg_read.php?t=0&m=0&s=1&d=1&mid=157&
PHPSESSID=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Where PHPSESSID is the user's current Session ID.
Copying and pasting this into a browser will allow you to gain access to that user's mailbox.