All messages being tagged NO_DNS_FOR_FROM on Zimbra server

On our Zimbra 6.0.5 server running on Fedora 6, it seems that all messages are being tagged by spamassassin with NO_DNS_FOR_FROM (despite having legitimate sent addresses).

How should I debug this? It started happening today...

Example header:

Return-Path: [email protected]
Received: from zzzzzz.yyyyyy.com (LHLO zzzzzz.yyyyyy.com) (NN.NNN.NN.NN) by
 zzzzzz.yyyyyy.com with LMTP; Wed, 1 Sep 2010 07:10:44 -0500 (CDT)
Received: from localhost (localhost.localdomain [127.0.0.1])
 by zzzzzz.yyyyyy.com (Postfix) with ESMTP id 84983B90D2D
 for ; Wed,  1 Sep 2010 07:10:42 -0500 (CDT)
X-Virus-Scanned: amavisd-new at zzzzzz.yyyyyy.com
X-Spam-Flag: NO
X-Spam-Score: -1.851
X-Spam-Level: 
X-Spam-Status: No, score=-1.851 tagged_above=-10 required=3.2
 tests=[AWL=-0.748, BAYES_00=-2.599, NO_DNS_FOR_FROM=1.496]
 autolearn=no
Received: from zzzzzz.yyyyyy.com ([127.0.0.1])
 by localhost (zzzzzz.yyyyyy.com [127.0.0.1]) (amavisd-new, port 10024)
 with ESMTP id 7rHCzaW15+sm; Wed,  1 Sep 2010 07:10:37 -0500 (CDT)
Received: from dm-mailman01.mozilla.org (mail.mozilla.org [63.245.208.162])
 by zzzzzz.yyyyyy.com (Postfix) with ESMTP id 0389EB90D24
 for ; Wed,  1 Sep 2010 07:10:37 -0500 (CDT)
Received: from dm-mailman01.mozilla.org (localhost.localdomain [127.0.0.1])
 by dm-mailman01.mozilla.org (Postfix) with ESMTP id 6F52D93E596
 for ; Wed,  1 Sep 2010 05:10:31 -0700 (PDT)
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Subject: mozilla.org mailing list memberships reminder
From: [email protected]
To: [email protected]
X-No-Archive: yes
Message-ID: 
Date: Wed, 01 Sep 2010 05:09:29 -0700
Precedence: bulk
X-BeenThere: [email protected]
X-Mailman-Version: 2.1.12
List-Id: 
X-List-Administrivia: yes
Sender: [email protected]
Errors-To: [email protected]