Hi Matt,
Any outbound Idoc uses "Port" to be sent to the outbound world. And the "port" is linked to RFC destination ( Transaction SM59 ) when we try to sent it to outside.
Now RFC destination links the Gateway or communicate via TCP/IP to the external system.
I suspect there are some issues with your RFC destination. Normally it is a basis activity but if you have authorization please check the RFC destination works first in the standalone mode or not. By doing a Connection test and Authorization test.
You can get lot of information over the internet how to do that. Once you sort that out we can look for the next error (if any )
R