Guestbook notification issue

Another variant of this problem - or probably another, similar problem?

The same versions of Guestbook and Drupal, PHP 4.3.10-19, Apache 2.0.54, MySQL 4.0.24, running as test system on a virtual host.

I don't get such error messages, but whenever I (as authenticated user) post a new entry to the website guestbook or a user's guestbook (both the same problem), then I (as admin) get an email from the mailer deamon of the real host system:

Subject: failure notice

Hi. This is the qmail-send program at myhost.de.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<entry@myhost.de>:
This address no longer accepts mail.

<guestbook@myhost.de>:
This address no longer accepts mail.

<new@myhost.de>:
This address no longer accepts mail.

--- Below this line is a copy of the message.

Return-Path: <system@mydomain.de>
Received: (qmail 30475 invoked by uid 33); 6 Jul 2008 12:20:27 +0200
Date: 6 Jul 2008 12:20:27 +0200
Message-ID: <20080706102027.30472.qmail@myhost.de>
To: New@myhost.de, guestbook@myhost.de,
  entry@myhost.de
Subject: 
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed; delsp=yes
Content-Transfer-Encoding: 8Bit
X-Mailer: Drupal
Errors-To: system@mydomain.de
Sender: system@mydomain.de
Reply-To: system@mydomain.de
From: system@mydomain.de
X-PHP-Mail: Sun Jul  6 12:20:27 CEST 2008 - /var/www/vhosts/mydomain.de/subdomains/cms/httpdocs - www-data x 33 33   www-data /var/www /bin/sh

By the way, the entries are perfectly stored int the guestbooks.

We have the same problem. no notification email is sent ...

Same problem here (http://drupal.org/node/277381)...

Marking as duplicate of #290722: Serious E-Mail Issues