Ok, I will simplify the question.
Docker container_a with a service configured to use Docker network_a needs to talk to container_b (discourse) using the internal Docker network. With docker compose I would simply add network_a to the networks configured for the service in container_b and set it as external.
I could reconfigure and rebuild my mail server but this is working configured as above for all my other docker container services I would rather have it working the same way for Discourse.