Remote Users and Outlook

I have several remote users using Win 98. I have an WINNT server with Exchange. They use TCP/IP as the protocol and Dial Up Networking and a 56k modem to log on to the server. The users are able to connect to the Exchange server with no problems. My problem is once they are on and try to bring up their mail, it takes from 5 to 15 minutes to open up. Is there any type of configuration I am missing for the system to retrieve their messages faster?