It's because the postfix docker container is not connected to a docker network that has access to the "lemmy" or "lemmy-ui" container, it's being connected to the "default" docker network. I submitted a pull request for it here that should fix it.

To make it work in the meantime:

1. cd into the Lemmy install directory and run docker compose down

2. Edit docker-compose.yml in the same directory, and in the postfix section, put this just below the postfix: line:

    networks:
      - lemmyinternal
      - lemmyexternalproxy
   

3. Run docker compose up -d

The indentation of that code is very important. Your postfix section should look like this when it's done:

That should connect the "postfix" container in to a docker network that can communicate with the "lemmy" and "lemmy" UI containers. There's another bug in the default config that doesn't assign a hostname to all the containers, but it doesn't always manifest all the time. You can fix that by making sure each service has a hostname assigned to it, like hostname: lemmy, hostname: lemmy-ui, hostname: postfix etc in the respective service's section of the service: section of the docker compose file.