Hi folks, let me explain my setup here. I have a Suse11 box with 2 network cards:
Eth1: 10.0.0.222
eth3 172.16.0.100
eth3:1 192.168.1.100
eth3:2 100.50.0.100
eth3:3 192.168.0.100

I have squid as a proxy on the Suse box, and with the default firewall I have to enable masquerading to allow clients on the eth3:1-3 to send and receive mail through the Suse box. I found the Suse firewall completely inadequate (all P2P software/connections are allowed once you enable masquerading) and had to install ConfigServer Security & Firewall. In die configuration of csf I could get my way around getting smtp to work for the eth3:1-3 clients, but pop3 connections does not go through the box. I know I need to allow port 110 and 995 to masquerade of NAT (or something) … and then the same for port 22

Im not entirely sure what the question here is...

show your iptables configuration. This will helps us to troubleshoot your problem. Till the time you can apply following firewall rules to allow mail traffic:

iptables -t nat -I POSTROUTING -p tcp --dport 110 -o <your internet interface> -j MASQUERADE
iptables -t nat -I POSTROUTING -p tcp --dport 25 -o <your internet interface> -j MASQUERADE
iptables -t nat -I POSTROUTING -p tcp --dport 995 -o <your internet interface> -j MASQUERADE

now check and save iptables.