The fact that you cannot receive mail could be related to several things. I usally take a stepwise approach.
On the LAN check if you can telnet to your server on port 25
If you can't then you need to check the server config first to make sure that the smtp service is running (Control Panel). Check documentation; there are a number of services that need to be running. By default some of the mail services are not runnning.
If you can and the server listening on port 25, then you need to check the firewall if its NAT is setup to point to the server. If it is then you should be able to telnet to the external IP of your firewall on port 25 and get the same reply as you did inside the LAN.
Hope this helps.
