Blocking internet access to workstations

We have a small network running NT4 server. I am trying to block some workstations from the internet while still allowing e-mail. There is probably a real simple solution but I have yet to find it. Any help would be appreciated.
We have one domain running several servers. The DHCP server is running NT4. We have a broadband cable connection running through a linksys router. The workstations are running windows 95. Through the router config I can block an I.P. range but that also blocks the e-mail access.