Server Admin Expert To Troubleshoot Email Software Problem

I have a dedicated server that is running a email campaign software application. The problem is that spam is being sent through my server and when I go to send out my own emails they will not send. I would like to find out how the spam is coming through and then prevent that from happening. The e-mails are being created by apache, which is the web server, so it's possible there is a broken script somewhere on one of the websites being hosted by the server that is allowing someone to spam through it. I have an email in queve on the server but it will not go through... Thank you for your bids.