[exim] Hide or rewrite particular flags in the header.

Does somebody knows how to configure exim to rewrite or hide certain
parts of the SMTP header in a message as for example:

An internal smtp server (using a private ip) use an certain Relay server
for outgoing delivery. Once the mail delivered, you can see the Internal
Private IP in the Header HELO part.

Received: from [192.168.xxx.xxx] (helo=ex.amp3.le) # here's the ip and
the server name that I want to rewrite or hide
    by ex2.amp2.le2 with esmtp (Exim 4.xx)
    (envelope-from <id@???>)
    id 1HcK8S-0001T1-IK
    for id2@???; Fri, 13 Apr 2007 13:40:25 +0200

Could it be maybe possible to rewrite such flags for particular hosts
using on a lsearch ?



Thank you.