Selective Masquerading, qmail, Return-Path, oh dear!

[Kezza]

How is user masquerading set up for remote destinations only?

If I'm bilbo@shire and I want to send mail locally to frodo@shire then qmail works like a charm. But what about when bilbo@shire wants to send an email into cyberspace masquerading as johndoe@isp.com? Changing my_hdr in muttrc for bilbo won't work, as:

1) local mail sent to frodo is now addressed incorrectly:

Return-Path: <bilbo@shire>
Delivered-To: frodo@shire
Received: (qmail 6557 invoked by uid 1001); 12 Dec 2002 10:47:02 -0000
Date: Thu, 12 Dec 2002 23:47:02 +1300
From: John Doe <johndoe@isp.com> <========== incorrect
To: frodo@shire
Subject: greetings from bilbo
Message-ID: <20021212104702.GB6432@shire>
References: <20021212104611.GA6544@shire>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20021212104611.GA6544@shire>
User-Agent: Mutt/1.4i

2) mail sent remotely cannot be delivered as isp.com cannot of course resolve the Return-Path bilbo@shire :

Return-Path: <bilbo@shire> <========== cannot be resolved
Received: (qmail 6350 invoked by uid 1001); 12 Dec 2002 10:29:25 -0000
Date: Thu, 12 Dec 2002 23:29:25 +1300
From: John Doe <johndoe@isp.com>
To: someone@somewhere.com
Subject: greetings from John Doe
Message-ID: <20021212102925.GA6337@shire>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.4i

I'm assuming that qmail itself should be changing the Return-Path; Mutt just changes the From field. But how do I change the Return-Path? More specifically how do I change the Return-Path on mail only sent to a remote host?