system.util.sendMessage to remote gateway

Hello everyone.

Is it possible to use the system.util.sendMessage script to communicate between remote gateways that are not configured as a Gateway Network? The only examples I can find use a Gateway Network.

If this is possible, what is the syntax required for the remoteServers parameter? I have tried the following:
remoteServers = [“x.x.x.x:8088”]

But I get this response:
[type=Remote Gateway,address=x.x.x.x:8088,sendStatus=UNKNOWN_GATEWAY]

Do I need to open a firewall port?

My message handler is on a remote gateway with a public IP address hosted on a webserver. I have been using the script console to test the sendMessage script from a local install. Both are running a demo license for testing.

Any advice is appreciated.

The Gateway Network is required.

Thank you for the reply.