Prevent server being used as a spam gateway

Our Centos 5 server hosts a couple of hundred domains, and several hundred mailboxes, all using authenticated SMTP.

Recently, our maillogs have grown to almost 1GB / day! On further investigation there are 20,000 messages in the mail queue, all of them outgoing spam.

Centos 5 running Webmin / Virtualmin
Postfix
Dovecot

I need to find the cause of the spam, because our IP is now blacklisted. I don't know if it is coming through a mailbox, or an insecure web script.

redcarrotAsked:
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BlazCommented:
Did you check the logs? If you have a sample spam message in the queue you can check the logs for the mail identifier and see where it originated.
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