Spam relaying via Plesk postix Mail Server

[PaddingtonC]

I have a Plesk 9.5 mail server on Ubuntu 8.04 LTS which forwards all SMTP traffic to an anti-spam server cacti.snowbarre.co.za. Many times I see the headers on the anti-spam server to contain from addresses not hosted on the mail server and I have checked and confirmed that my server is not an open relay server. How can a spammer be using my server to relay spam traffic? How can I stop this?

Open relay test:
paddington@paddington-MS-7387:~$ telnet cm 25
Trying 196.201.x.x...
Connected to cm.
Escape character is '^]'.
220 cm.snowbarre.co.za ESMTP Postfix (Ubuntu)
mail from:test@facebook.com
250 2.1.0 Ok
rcptaddington@yahoo.co.uk
221 2.7.0 Error: I can break rules, too. Goodbye.
Connection closed by foreign host.
paddington@paddington-MS-7387:~$


A typical headers is:
View basic headers
Received from cm.snowbarre.co.za (cm.snowbarre.co.za[196.201.x.x]) by cacti.snowbarre.co.za (Postfix) with ESMTPS id 00B601881AD; Mon, 27 Aug 2012 14:03:29 +0200 (SAST)
Received from cm.snowbarre.co.za (localhost [127.0.0.1]) by cm.snowbarre.co.za (Postfix) with ESMTP id 81627367E007; Mon, 27 Aug 2012 14:02:50 +0200 (SAST)
Received from User (ml82.128.x.x.multilinksg.com [82.128.x.x]) by cm.snowbarre.co.za (Postfix) with ESMTP; Mon, 27 Aug 2012 14:02:49 +0200 (SAST)
Reply-To <amsnkeuri@ymail.com>
From "Ms Nkeuri Aguiyi"<admin@jogor.net>
Subject Your Unpaid Fund.
Date Mon, 27 Aug 2012 05:03:22 -0700
MIME-Version 1.0
Content-Type text/html; charset="Windows-1251"
Content-Transfer-Encoding 7bit
X-Priority 3
X-MSMail-Priority Normal
X-Mailer Microsoft Outlook Express 6.00.2600.0000
X-MimeOLE Produced By Microsoft MimeOLE V6.00.2600.0000
X-Antivirus avast! (VPS 120821-0, 08/21/2012), Outbound message
X-Antivirus-Status Clean
Message-Id <20120827120250.81627367E007@cm.snowbarre.co.za>
To undisclosed-recipients:;