When I installed 5.0.13_GA I used a live routable IP Address so I could understand and learn about this wonderful solution I learned about while working for H & R Block who uses it for email and I wanted to jump on this bandwagon. Once I had it installed and working without any problems and moved over some email accounts and showed them how to use it I left it run for approx 2 months without any issues.
Now since I received my new Hardware Firewall device, I changed the IP Address of this server from a routable ip address to an internal ip address. With the help of google I was able to get the server back up and running most of the way since this change. I am able to receive inbound email without any issues.
My issue at the present time is when sending out email from this machine, I am able to telnet external.mail.server 25 and receive a helo prompt for most email messages. On some of them like gmail.com I get a message about connection timed out port 25. I have not checked other commercial mail servers just the ones that have a deferred state in the system.