DETERMINE THE INTERNAL USER BEHIND FIREWALL

HI ALL,
I AM AN IT ADMINISTRATOR AND I AM USING A FIREWALL TO PROTECT THE INTERNAL USERS FROM INTERNET AND I USE PRIVATE IP FOR MY LOCAL NETWORK WHICH APPEARS FOR THE INTERNET AS A SINGLE REAL IP THE PROBLEM THAT I HAVE ONE OF THE LOCAL USERS WHO SENDS BAD AND MISUSE EMAILS FROM MY NETWORK AND I CANT DETERMINE WHO IS THAT LOCAL USER BECAUSE HE USES WEB MAIL LIKE YAHOO SO IS THERE ANY APPLIANCE OR SOFTWARE CAN BE USED TO DETECT THAT USER
THANK YOU ALL