ISA Server is separate from Exchange 5.5 server. What should the DNS and default gateway settings be on the Exchange 5.5 server?
Your DG should be set to the internal NIC address of your ISA Server.
Your DNS entries should be set to your closest upstream DNS servers. If you provide primary and secondary DNS for your organization, then use those addresses (even if they are inside of the firewall). However, if DNS is being supplied by an ISP, or other outside source, then use their addresses.
Also, be careful not to use the FIREWALL CLIENT on your MS Exchange Server. It conflicts with the Internet Mail Connector within Exchange.
Refer to Microsoft Knowledgebase article #Q181420 for full details on how to configure Exchange Server to sit behind the firewall.