I am in a similar psoition to you and this is how i have overcome the issue.
forward port 110 from the firewall to the ip address of the e-smith box, however, DO NOT FORWARD port 25 (smtp) to the e-smith box from the firewall as this may turn you into an OPEN RELAY box and cause you no end if grief, lest of which will be mail from your domain being rejected by other mail servers
hope this helps
Gavin