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

Code:
Received: from localhost (localhost.localdomain [127.0.0.1])
..to say

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

Code:
Received: from realhostname.mydomain.com ([127.0.0.1])
              by localhost (realhostname.mydomain.com [127.0.0.1])
..to this

Code:
Received: from realhostname.mydomain.com ([Public IP])
              by realhostname (realhostname.mydomain.com [Public IP])
Basically I don't want the terms localhost, localdomain or 127.0.0.1 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