Can you telnet to port 110 externally for pop?
You will need to ensure pop is working locally via telnet <ip> 110.
Then you may need to port forward via firewall.
*** Improved by Wrobinson on 4/18/08 ***
Make sure that POP is enabled as a protocol at the server level in ESM, also that the user is configured to use POP in Active Directory. Lastly, ensure that the POP service is enabled and running on the server. Once all of these things have been confirmed, follow the instructions above, though you can start your tests internally to ensure that you can reach the server from the local LAN and then test from the Internet. If you can reach the server internally but not from the Internet, then chances are that the firewall needs to be configured to allow POP through.
You might also consider alternative solutions such as RPC/HTTP(S) or OWA.