Automatic email retreival

We are running Exchange 5.5 on an NT4 box. Clients are running Outlook 98 and OWA.

Question: Is there a way to have the exchange server automatically check and download emails from the POP3 server at our ISP at all times? Our system is doing this now, but only when the user is logged into the network (locally). We need for exchange to collect all of the emails for when an OWA session is started.

TIA,
Byron