In one of the offices, we have a 50-computers network, which had only one server machine:

• Windows 2003 Server
• Microsoft ISA Server
• Microsoft Exchange 2003

This server worked as a gateway (proxy server), mail server, file server, firewall and domain controller. It had two network interfaces, one for WAN (let's say 222.222.222.222) and one for LAN (192.168.1.1). I set up a Linux box to be the gateway (without a proxy), so the Linux box now has the following interfaces: 222.222.222.222 (our external IP, we removed it from the Windows machine) and 192.168.1.100 (internal IP), but we need to keep the old Windows server as a mail server and a proxy for some of our users, until we prepare another Linux machine for that, so I need the mail server on that machine to be available from the Internet. I set up iptables rules to redirect all the incoming connections on the 25th and 110th ports of our external IP to 192.168.1.1:25 and 192.168.1.1:110 and when I try to telnet our SMTP service

 telnet 222.222.222.222 25

I get the greetings from our windows server's (192.168.1.1) SMTP service, and that's works fine. But when I telnet POP3 service

 telnet 222.222.222.222 110

I only get the blank black screen and the connection seem to disappear if I press any button. I've checked the ISA rules - everything seems to be the same for 110th and 25th ports. When I telnet on 110th ports of our Windows server from our new gateway machine like this:

 telnet 192.168.1.1 110

I get the acces to it's POP3 service:

+OK Microsoft Exchange Server 2003 POP3 server version 6.5.7638.1 
(...) ready.

What sould I do, to make the POP3 service available through our new gateway?