Restricting outbound DNS access on Win98

Background: Email server being hosted by external company on public IP address. We have no internal DNS server, but use our ISP’s public DNS servers ip addresses.

Problem: I have a couple of PC’s that need access to the Email server using Outlook/Outlook Express, but I don’t want them to have full access to the internet. Ideally, I would like to have a simple solution, such as including the ip address for the mail server in the hosts file restricting them to only access that public ip address. Currently they have the DNS server addresses in their tcp/ip networking properties, and they have full access.

Is there a way to effectivly restrict these computers to a single public ip resolution, so that they can retrieve email only?