you might be out of luck.
Exchange is a mail server, most of the POP3 settings are for clients not the server. The only way to retrieve POP3 mail is if it is a Small Business Server (sbs) which has a POP3 connector for doing this.
If you haveSBS then if it is 4.5 download the POP3 connector and install.
If it is a plain Exchange server, or in general it is better to have your domain mail sent directly to your Exchange server. This would be contacting the ISP and having them change the MX record if you have a static IP address. If you don't have a static IP then you should look at a store-n-forward service from an ISP, it uses the ETRN command. Basically the ISP would collect all mail for the domain and when you server connects it would issue the ETRN and then recieve all the mail and route it to individual mailboxes.
If this is confusing email me and I'll explain