I have just configured Centos 5.5 LocalMailServer with fetchmail and sendmail , Proxy with Squid and FileServer with samba. Now my concern is security.. How can i protect my server with outside attack. Will I need to block some ports or I need special tools or script so no one from outside can attack my machine.

My machine is working on intranet with local ip only.. No web server or static ip exists. Machine is connected with ADSL router to access internet.

Thanks in advance

If machine is not exposed to external world, your system is not at risk from outsiders but insiders itself. If you are connecting to internet using adsl modem, you can block certain ports at that level.
iptables with tcp wrappers should make your system stronger. Blocking ports is one way to secure your system but you need to also keep watch on the processes running.
But your question is very vague to answer it to the point. Can you elaborate what you want to do with the server? How is it going to be used? Who are going to be the users or clients?

Just to add a point of clarification on the last post and the original question: The server is on local LAN IP only, but connects to the Internet via an ASDL router. If you have any port forwarding in the router then the services on these ports are exposed to attack almost the same degree as if the machine were directly connected. I concur with Linuxlover.chaitanya, please elaborate on your objectives so that we can better advice with regards to security recommendations.

I missed out on the port forwarding part. And if that is done, then server is at risk. OP needs to clarify on that part.

Thanks for valuable suggestions from all of you. I do not have any port forwarding in my ADSL router right now but in later stages I would like to do it also.

If you are planning on have port forwarding for putting your server live, then you need to describe what services are you going to provide your clients. What is going to be the purpose of your server? What do you want to do with it?