maybe something like this (going from memory, may need to check position, switches, ... for accuracy if you get any errors):
Code:
iptables -t nat -I PREROUTING -i ppp0 -p tcp --dport 110 -j DNAT --to 192.168.0.2:110
iptables -I FORWARD -i ppp0 -d 192.168.0.2 -p tcp --dport 110 -j ACCEPT
you'll also want a typical "RELATED,ESTABLISHED" ACCEPT rule, the ip_conntrack module(s), and have enabled ip forwarding.
Since the linux router "owns" both the IP on ppp0 and eth0 it should respond to your clients on the LAN regardless of the interface that the request comes in on.
Hope this helps.