Thanks... it does appear port 25 is blocked. However, our ISP itself doesn't block any of our ports (I've had this conversation with them in the past).
Also, both servers are connected to the same router, with nothing in the hardware firewall. If the firewall isn't on the router, where else might I look?
FYI: using http://network-tools.com/
I get a connection (that's rejected because relaying isn't allowed)