When using POP3 you generally retrieve all messages off the server and store them on the user's PC. No email is then kept on the mail server, it can be found on the computer that accessed the account. Forwarding sends the email to the users address, which has a "forwarding" setup for that users box. It will then send the email to the address setup in the users "forwarding" settings. It could be forwarded to one email address or to several other addresses.