[exim] trusted_users option has stopped working for me

I'm up to Exim 4.71 with Ubuntu Lucid.

I've got some procmail jobs that send out email using Exim, as a named
system user.

:0
|/usr/sbin/exim4 -f liaison@??? $SUBSCRIBER

In this example, the user is liaison

liaison:x:1007:1007:liaison,,,:/home/liaison:/bin/bash

The body of the email being sent out contains headers like this

>From Debian-exim Thu Nov 3 02:27:45 2011
Date: Wed, 02 Nov 2011 22:27:26 EDT
MIME-Version: 1.0
Message-ID: <201111022227.7d904eb1fc1212e@???>
Subject:
From: liaison@???
To: xxxxx@???
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit


That did not use to happen.

The Exim Configuration file contains

trusted_users = "Debian-exim:liaison:listmast"

Is this a new security feature introduced recently ? Something that I
have to enable when compiling Exim ?

Got any troubleshooting tips for me ?

Thanks

mark