I also had another question regarding the above: What is the rule in ISA 2004 Standard Edition that allows me to join the network and retrieve the users and groups from the Exchange Server?
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To gain access to network resources, configure your ISA to act as a VPN server that will enable a valid authenticated client to access authorized resources including managing users and groups.
For detailed guide on how to achieve this, please refer to Microsoft's Technet site.
This was first published in November 2008