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Based on your description, ISA Server is acting as a proxy rather than a firewall, and you are relying on your DSL modem's built-in firewall to protect your network. In my experience, firewalls built into DSL modems do not usually provide the level of management granularity necessary to properly configure it for a network.
I recommend one of the two following options:
- Add a second NIC to Windows SBS and configure ISA Server as a firewall in addition to a proxy.
- Invest in a firewall appliance such as a SonicWall TZ180. Configure the appliance for your network and disable the firewall feature on the DSL modem.
If you choose the ISA Server option, you can use the Windows SBS wizard to configure ISA Server to permit inbound email and/or other services.
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