If you want to use VPOP3 through a firewall, you will need to open up some TCP ports in the firewall.
VPOP3 uses several ports, for the different services it provides. The best way to see the ports it uses is to go to Services → General in the VPOP3 settings. This shows the various services, and which ports it uses.
The defaults are:
Obviously all these port numbers can be changed from the defaults, so going to Services → General will let you see what ports you have configured VPOP3 to use.