If you can modify your firewall settings :
You must activate acces from the clients to internet for the following ports : 25 and 110
If you are using iptables the following rules are what i use :
iptables -A FORWARD -i laninterface -o externalinterface -p tcp
--dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT
As well as the same for port 110.
Hope this helps or gets you on the right track.
if anyone know how to connect to the pop mail server hosted in internet using outlook express, but the client can access internet only by using squid proxy server.
Please help me on this matter.