I don't really understand what you want to rewrite? What have to be re-written, when the node sends a POST to third-party? Can you give more explanation of this?
A HTTP Code 400 is a Bad Request. This is typically seen when the destination server uses HTTPS, but the client didn't initiate an SSL Handshake. There are much more reasons for 400 responses, this was just an example, which may match, because you wrote something about Port 80, which is usually HTTP and I'm pretty sure that Salesforce does not support unencrypted HTTP traffic.