Hide firewall ?

[Mike A. Sauvain]

hyall, i need to hide the internal ip of my mailserver,
it see them only in messages, where i send and recive on my host:

Return-Path: <[EMAIL PROTECTED]>
Delivered-To: [EMAIL PROTECTED]
Received: (qmail 2817 invoked from network); 9 Apr 2001 02:40:18 -0000
Received: from 62.x.x.x (HELO client) ()
  by 192.168.1.20 with SMTP; 9 Apr 2001 02:40:18 -0000
 ^^^^^^^^^ ^^ ^^^
how i can change them ?

thanks for advice ..*