Linux Router won't route back from email server
 

I am setting up some linux firewall boxes (to make some gig firewalls and save money) (running Linux 2.4.20 Slackware 9.0 using iptables). I set up a private network behind the firewall, and I have them working with everything on the network (web, dns, ssh, ...), but for some reason, they are not working with our mail server. When I run Outlook Express or Eudora (another email app) on the private network (behind the firewall), they will send and receive mail, but then they get stuck in trying to "close connection with mail server", then eventually they timeout and say that they couldn't close the connection, even though they will send and receive mail just fine. This led me to believe that I was just missing a rule or two in my iptables (in addition to accepting POP3 and SMTP), so I set up some logging to look at the dropped traffic, but found nothing. So I decided to simply change the FORWARD rule to ACCEPT everything, and it didn't make a difference. Which makes me believe that the iptables rulesets aren't the issue. Everything else gets forwarded to the private network (and vice-versa) just fine, but for some reason these mail apps aren't working (completely...). This makes me believe that it isn't a port I am missing on my rules, but something is wrong with the routing, or some module or something else that I obviously don't know that I need is missing. I am just curious if there is some module or configuration I need to add to do this. The default routes created seem to be working fine for everything else, but something just isn't getting back from this handshake that obviously goes on after the mail is sent from the server to the client app. I need to get this figured out, for the simple fact that I imagine I will only discover more problems like this one once I start trying some other apps. The mail server is a Sun Solaris box running iPlanet), and the linux box is running on a P3 800 with two 3com gig nics (that all appear to be working great). I really am clueless on why everything else will work (especially when I ACCEPT everything), but this little thing with the mail apps won't work.
I would appreciate any help, or suggestions on where to even look (email server, linux box, help sites ....anything), I haven't found any literature pertaining to this problem, or any like it. Any recommendations or stories would be great if any of you have set something like this up before.
Thanks in advance,