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
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.