Using Postman to test remote connectivity

I’m using Postman to test connectivity to a vendor implemented service. They sent me sample JSON body of a request they are able use to successfully POST to the service. I call the same service remotely with the exact same JSON, but I’m getting a ‘500 Internal Service Error’.

They also have a test ‘Hello World’ service available that I am able to successfully call.

The request I’m sending is raw JSON (224 KB). The TCP connection info provided by Postman is in included below. Any ideas where I can go from here?