I host an e-mail server running the Fedora Core 5 operating system and the Postfix e-mail server. I have Postfix configured to require authentication for outgoing messages and to only allow messages from my domain (no open relay). That said, over the past few days, I've noticed an onslaught of spam being sent through my server.
My question is... considering the above configuration, is it possible that a computer on my network is infected with a virus/worm/trojan that is doing it, or is it possible that some spammer got clever and found a way around the configuration? There are roughly 150 computers on my network - all Windows-based PC's.
In addition, what steps can I take to stop my server from sending spam, other than the Postfix configuration file since that obviously isn't effective.
Thanks for your help.