Spammer relaying via Postfix mail server

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http://serverfault.com – I have a Plesk 9.5 mail server (cm.snowbarre.co.za) 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) ma ()