I have a Mandrake Linux 9.1 server running samba, squid and iptables. The clients are windows 9x. The clients need to send mail through my isp's smtp server and receive mail through the isp's pop3 server. When they try to send or receive mail, they get a socket error. I tried shutting down iptables, but it does not work. All other things are working fine. What could be the problem? Do I need to configure iptables?

The details of my server are :
MDK 9.1
1 NIC 10/100MBPS ip address 192.168.0.2
A dsl connection
kernel 2.4.21-0.13mdk

The clients need to be able to access the Internet in some manner. Are the clients able to browse the Internet without Squid? Actually a better question, can the clients all ping addresses on the Internet, such as www.google.com? If the clients can access the Internet, then you just configure their e-mail clients to talk directly to your ISP's SMTP and POP server.

If the clients cannot access the Internet, then you need to provide access some how, such as through a dual-homed box that does ip-masquerade.

No. The clients cannot connect to the internet without squid. I do not know how to setup a dual homed box that does masquerading. Any pointers?

Check out the FAQs in the Security forum that can be found in the Security References post by unSpawn. It's a stickied post near the top of the forum, there are many, many HOW-TOs and FAQs on IP masquerading/NAT.