ok so do you want to provifr a proxy service through your gateway or do you want to retreive the mail from other pop servers and combine them all on your server in one central account?
if you want to run a proxy service them a simple firewall only letting the pop3 requests through would do this effictively. or in fact you could just set up a basic firewall at all and have this capability.
check out
http://netfilter.samba.org for iptables setup info
If you want to retreive mail back for them and deliver it into a central account have a look at fetchmail
Rich