How can I change this section of my outgoing/incoming message headers:

Received: from localhost (localhost.localdomain []) say

Received: from realhostname ( [Public-IP])
Also the amavisd section of the headers I want changed from:

Received: from ([])
              by localhost ( []) this

Received: from ([Public IP])
              by realhostname ( [Public IP])
Basically I don't want the terms localhost, localdomain or in the message headers anywhere.

This is easy to do with Apache James, I just need to know how to do it with Zimbra/Postfix.

Thanks for your help