I'm running ICS from my Win2k Advanced Server. I have a 98 box connected to it. The 98 machine is able to get pages, Telnet, FTP, and even get mail. However, it is unable to send mail. I have tried disabling all servers on the 2000 machine. It is set as a workgroup, not a domain. The 98 machine can send fine after switching it from the network settings, and putting it on it's own IP, so I know that the mail servers are in there right. It cannot reach the SMTP server by using the default of mail or putting in the SMTP server IP. I can ping the SMTP server IP, but cannot send mail through it. I know it has to be a simple setting somewher, but I'm at a loss as to where it could be. Please help me from pulling my hair out.
This sounds like a DNS problem. When you ping the SMTP server, do you ping the IP address or the name? Try pinging the name. If that fails, you know you have a name resolution problem.
The client machines obtain their DNS settings from the ICS gateway, which in turn gets the information from the service provider. It's possible that this is not working for some reason. Run Winipcfg at the W98 machine and see if you have a DNS server then ping the DNS server to see if it is available.
Also make sure you haven't overridden the IP entries at the client. Leave all entries blank for DHCP.
Finally, check to see if you are making assumptions about a DNS suffix at the client. For instance, when the client is physically connected to the Internet interface, it may get a configuration packet that includes a DNS suffix such as "router.anotherrouter.serviceprovider.com". But when the client is behind the ICS server, it will not get this suffix. By modifying the name of the SMTP server in your mail client with the full name of the mail server including the suffix, you should be able to resolve the name.