Exchange 2000 E-mail carries Private IP

We have recently deployed exchange 2000 Internet based E-Mail System and it’s behind the Firewall.
When I send a E-Mail from a Client PC it’s carries the private IP address of the PC which I’ve send the E-Mail (in the header record). Pls. help me to resolve this problem & it’s sholud only reveal the Public IP to others.

Regards.

Shiran – System Supports