Undelivered Mail Returned to Sender

Hi, once in a while we are getting bunch of failure notices such as message below. It is a spam. Our email addresses are on our website: http://www.aeispeakers.com/about.php. How can I stop or reduce those emails?

From: MAILER-DAEMON@mx-nrs5.iad.mail-abuse.org (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: olga@aeispeakers.com


This is the mail system at host mx-nrs5.iad.mail-abuse.org.


I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.


For further assistance, please send mail to postmaster.


If you do so, please include this problem report. You can
delete your own text from the attached returned message.


                   The mail system


<cadizht@shakopee.k12.mn.us>: host 204.169.209.230[204.169.209.230] said: 550
    5.1.1 User unknown (in reply to RCPT TO command)


Reporting-MTA: dns; mx-nrs5.iad.mail-abuse.org
X-Postfix-Queue-ID: BBB164C3042
X-Postfix-Sender: rfc822; olga@aeispeakers.com
Arrival-Date: Sat, 14 Jun 2008 22:49:18 -0400 (EDT)


Final-Recipient: rfc822; cadizht@shakopee.k12.mn.us
Original-Recipient: rfc822;cadizht@shakopee.k12.mn.us
Action: failed
Status: 5.1.1
Remote-MTA: dns; 204.169.209.230
Diagnostic-Code: smtp; 550 5.1.1 User unknown


Received: from 121.20.141.90 (unknown [121.20.141.90])
        by mx-nrs5.iad.mail-abuse.org (Postfix) with ESMTP id BBB164C3042
        for <cadizht@shakopee.k12.mn.us>; Sat, 14 Jun 2008 22:49:18 -0400 (EDT)
Message-ID: <000a01c8ce92$016c0a5f$33d21faf@cxkes>
From: "Replica Watch Dealer " <olga@aeispeakers.com>
To: "Quality watches" <cadizht@shakopee.k12.mn.us>
Subject: Replica Handbags
Date: Sun, 15 Jun 2008 01:01:26 +0000
MIME-Version: 1.0
Content-Type: multipart/alternative;
        boundary="----=_NextPart_000_0007_01C8CE92.016914CD"
X-Priority: 3
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook Express 6.00.2900.3138
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3198