qmail smtp abuse despite rcpthosts file

Dear all,

I'm running a qmail server installation which has been subject to abuse
since a few days. I have an rcpthosts file, and relaying is disabled.
The following log excerpt shows an smtp transaction, successfully
sending a spam email. That email bounced, and was therefore forwarded
back to me. The non existing recipient (bducb@mangm.de) appears as the
*sender* of the spam email (mail header pasted below) and any spam
emails sent through my system seem to share this syndrome.

Can somebody kindly explain the mechanism used for successfully sending
these messages, although relaying is disabled and a rcpthosts file
exists?
Incidentally, I recently transferred the domain "mangm.de" from a
windows server running Ability Mail Server to a server running
Linux/qmail.

On Ability Mail Server, I had used implicit tsl for smtp
authentification and had disabled relaying, but was successfully
subjected to the same kind of abusive mails nonetheless.

LOG EXCERPT:
Oct 15 12:31:21 h772330 qmail: 1160908281.479282 new msg 2851733
Oct 15 12:31:21 h772330 qmail: 1160908281.479359 info msg 2851733:
bytes 18841 from <> qp 32156 uid 2020
Oct 15 12:31:21 h772330 qmail: 1160908281.490857 starting delivery 707:
msg 2851733 to local 7-bducb@mangm.de
Oct 15 12:31:21 h772330 qmail: 1160908281.490899 status: local 1/10
remote 0/20

ACTUAL MAIL HEADER:
Return-Path: <bducb@mangm.de>
Received: from e182121160.adsl.alicedsl.de (e182121160.adsl.alicedsl.de
[85.182.121.160])
by six.baremetal.com (8.13.4/8.13.4) with SMTP id k9FAQAss012318
for <webmaster@creativepursuits.ca>; Sun, 15 Oct 2006 03:26:11 -0700
Received: from 85.182.239.53 ([85.182.239.53])
by e182121160.adsl.alicedsl.de (8.13.4/8.13.4) with SMTP id
k9FAZcTN062681;
Sun, 15 Oct 2006 12:35:38 +0200
Message-ID: <45320DA9.5000200@mangm.de>
Date: Sun, 15 Oct 2006 12:30:01 +0200
From: Benny Cameron <bducb@mangm.de>
User-Agent: Thunderbird 1.5.0.7 (Windows/20060909)
MIME-Version: 1.0
To: webmaster@creativepursuits.ca
Subject: liability heavy-duty
Content-Type: multipart/related;
boundary="------------000501040502080909050803"
X-Scanned-By: MIMEDefang 2.36