I have a mail server using CenOs 5 as Os and postfix as an MTA. My mail server is used for sending mails of clients subscribed to send mail from my server.The problem is that some of my clients are sending spam from my mail server,is there any way to trace he person who is sending spam by using my mail server before he/she sends a mail from my mail server.
Firstly make sure you're not an open proxy.
There are a number of checking systems. Google "open proxy check".
Then, if you have some sample spam messages, check that SpamCop suggests to send reports to you.
I don't know your specific system to suggest how to snoop messages to check for spam before it is sent :-(