You can configure a POP3 Connector on your Exchange server. This will download messages from your ISPs email server and populate the messages in the Exchange mailboxes. If your Exchange email accounts are setup the same (same email address and domain name) as the ISP, you can send from these accounts, as well.
Try searching POP3 Connector for Exchange on the Web. There is a ton of information available.
<a href=”http://www.gravityusa.com/phoenix”>Gravity Systems</a>