Replace Private IP by Server Hostname in mail header

[Rachid Abdelkhalak]

Hello List,
I have a mail relay and an internal mail server both under Postfix and 
behind a firewall (DMZ and LAN), on both segment i'm using a private IP 
address with NAT.

On all outgoing emails headers sent by our users, i can see my servers ip 
addresses (private).

Is there any config that i can do to make postfix write hostname instead 
of the ip address on the header or replace the private ip address by the 
public ip address?

Thank you

Brest regards.