We have two Macs behind a SBS ISA installation. I have two issues, one is the same issue that you are having. The other is that one of the guys has DSL at his home and wants to check his email. Rather than opening POP ports, I would prefer that he VPN into the firewall. Does anyone know of a VPN Client for Fruits (sorry, Apples)? Also, when the MACs access the internet, they are prompted for a password with each connection.
We have two Macs and we can allow them to surf by configuring their system a web client. Other than that, we cannot use FTP or any other protocols. Can't even setup the Macs as SecureNAT client ;-( If you guys have an answer, pls let me know.