Delivery problem with empty recipient

Hi,

I have a delivery problem on a webmail server with postfix and horde/imp.
Sometimes users sends mails with an empty recipient like ""@localhost.
Unfortunatly postfix deliveres these mails to root@localhost. Why?
How can i bounce mails to ""@localhost with an "user unknown error"?

With kind regards,
Konrad Mauz

(anonymized) mail header:
Return-Path: ValidFromAdress@fh-konstanz.de
Received: from outwebmailhost.fh-konstanz.de
(outwebmailhost.fh-konstanz.de [141.37.x.x])
by mailhost.fh-konstanz.de with ESMTP id
j0SHB5DE028281 for <root@fh-konstanz.de>; Fri, 28 Jan 2005
18:11:05 +0100 (CET)
Received: by ourwebmailhost.fh-konstanz.de (Postfix)
id 9263517E23; Fri, 28 Jan 2005 18:11:05 +0100 (CET)
Delivered-To: root@ourwebmailhost.fh-konstanz.de Received: from localhost
(localhost [127.0.0.1])
by outwebmailhost.fh-konstanz.de (Postfix) with ESMTP id 8583817E22 for
<""@localhost>; Fri, 28 Jan 2005 18:11:05 +0100 (CET)
Message-ID: <1106932265.41fa722959356@ourwebmailhost.fh-konstanz.de>
Date: Fri, 28 Jan 2005 18:11:05 +0100
From: ValidFromAdress@fh-konstanz.de
To:
Subject:
MIME-Version: 1.0 Content-Type:text/plain
Content-Transfer-Encoding: 7bit
User-Agent: Internet Messaging Program (IMP) 3.2.2




--
Konrad Mauz Rechenzentrum Fachhochschule
Braunegger-Strasse 55, D 78462 Konstanz
e-mail: kmauz@fh-konstanz.de
Tel.: +49(0)7531-206-472