To send something outside of your network you use the same syntax as for inside your netowrk, which was:

net send

But in order for this to work the receiving computer must have windows messenger (not msn messenger) turned on. Windows 3.1, 95, 98, and ME DO NOT have this feature available. So no matter what you do, a box running any of those OS's can not receive a net send message. If you're gonnna send a message over the internet make sure you use the IP address of the computer you're sending too as opposed to the network name. Also, something like a firewall between you could stop the message going through.