Hi every one

I am running squid as a proxy server which is connected to net through
ppp0 my users in lan can browse net using this proxy but they cannot
access pop3/smtp mail using outlook express, from other posts I came to
know that i have to MASQUERADE to access pop3/smtp.

I tried fallowing commands to do this
1)
iptables -t nat -A POSTROUTING -p tcp -j MASQUERADE --to-ports 25
iptables -t nat -A POSTROUTING -p tcp -j MASQUERADE --to-ports 110
2)
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 25 -j ACCEPT
iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 110 -j ACCEPT
3)
iptables -A FORWARD -p tcp --dport 25 -j ACCEPT
iptables -A FORWARD -p tcp --dport 110 -j ACCEPT
4)
iptables -t nat -A POSTROUTING -p tcp --sport 25 -j ACCEPT
iptables -t nat -A POSTROUTING -p tcp --sport 110 -j ACCEPT
5)
iptables -A FORWARD -o ppp0 -p tcp --dport smtp -j ACCEPT
iptables -A FORWARD -o ppp0 -p tcp --dport pop3 -j ACCEPT

But non of them are working, but if I use

iptables -t nat -A POSTROUTING -o ppp0 -J MASQUERADE

then it is working ok, but in that case there is a chance that users
may access the Internet bypassing squid by disabling the proxy setting
in browser which i don't want, I only want to route pop3 & smtp to
route through MASQUERADE. Plz. Help.

Thank you all in advance.