Exchange 2000 (SBS 2000)

I'm running a SBS 2000 installation. It is a dual NICed server with a live IP address and a internal address range. Exchange is allowing internal mail and is able to receive mail, but cannot send mail. I thought that ISA might be blocking port 25, but I've got packet filters running for inbond and outbound mail on port 25, and also a server publishing rule to point the internal NIC to the external NIC. Every time a mail is sent a none delivery response is sent back in 1-2 days with 4.4.7 error.There are no errors in any of the logs. The only thing that I can see in the ISA logs is that every time it trys to connect to the smarthost to deliver the mail, it uses a random local port in the 8000 range and an external port of 25, even though Exchange is configured to use port 25 always. Why can't I send mail??? Please help.