What would be IP address of Email send on LAN

Management has asked me to trace the source machine or IP from which the email was send.
– i have Exchange 2003 and the sender & Recepiant are within the network so it has become difficult for me to trace the sender
i have tried looking at the Security Log but the Logs are configured to be overwritten everyday and Email to be traced is a week old.
– can the Email be traced from Antispam server even if its not a SPAM.
or……….

Help me out guys