Well, I'm using NT4 but it is still in TCP/IP properties somewhere but you might have to drill down a bit more. Or, you can do it with the registry. This is taken from the MS Knowledge Base ....
Enabling TCP/IP Forwarding
Use Registry Editor (Regedt32.exe) to view the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
Set the following registry value:
Value Name: IPEnableRouter
Value type: REG_DWORD
Value Data: 1
NOTE : A value of 1 enables TCP/IP forwarding for all network connections installed and used by this computer.
However, I doubt that your problem is with IP Forwarding if you can still get external access from a DOS prompt but it's just IE that's not working.
What about the POP server on your LAN. Can you reach it by IP address or host name. Can you telnet to it on port 110 or is it just your mail client that can't connect?
Chris.
************************
Chris Andrew, CCNA
chrisac@gmx.co.uk
************************