you have three different things here, none of them have much to do with the other
postfix is an smtp server. It's used for users and other allowed people to send mail through your server, and to it's users.
qpopper is a pop3 server. It's used for users on your system to retrieve their email that's stored on your server.
To retrieve mail from another pop3 server that you have an account on you will want to use fetchmail, or something like it.