POPFiles acts as a proxy for POP3 connections and it displays its user interface through the browser. By default, both the POP3 proxy and the http UI will only accept connections from the local machine and not from someone somewhere on the internet. That's what we call stealth mode.

However, if you need POPFile to accept connections from other machines, you can go to the security tab in the POPFile UI and disable stealth mode separately for both ports. If you don't need this, make sure you don't allow connections. If you allow connections, find some way (a firewall) to make sure that they don't come from outside your network.