I enabled pop3, and set the binding to 0.0.0.0:110, ie open to anyone (I think). When I connect to it from the local network with the local IP it works perfectly, but when i try to connect via the external IP from outside the company or even from within, the port seems closed. Whats wrong?

You should contact your Firewall staff and verify that they do not block port 110 Deli

I have double checked with the ISP that our firewall is open and forwarding to 110, I cant get into the firewall by myself but when I try to telnet to port 110 it responds immidiately that the port is closed, ALL other ports take a few seconds to come back with that responce.

Do you have a firewall enabled on your server? Maybe it only accepts local IP to connect to the port? How did you set bindings on the POP3 110 port? Did you use Exchange System Manager and set IP Address to (All Unassigned)? Deli

The server is a basic Win2003 Enpterprise R2, so besides any default MS security measures there is nothing special about it. I have checked and the windows firewall isnt even running. We run OWA on that server as well, so port 443 on the exact same machine is fine. I set the port binding with the cmdlets. set-popsettings -unencryptedortslbinding "0.0.0.0:110"Whre in 2007 do I find the System Manager?

Ok I was assuming that you were using Exchange 2003 So you are using Exchange 2007 so no system manager but Exchange Management Console My opinion is that something is blocking your port from the internet, a little bit hard to troubleshoot if you cannot access the router/firewall, did you verify that the IP the port is forwarded to is correct? You client does not block outgoing connections on port 110 (client firewall) Deli

After a bit of a battle it turns out the ISP was lying to me, after the 100th call and escalation they reopened the port and redid the NATing and all works now. Thanks!!!