Configuration is located at data/postal after the first start. Note that non-config files (i.e. files in https://github.com/atech/postal/tree/master/config) will be overwritten each time the container starts up. See https://github.com/atech/postal/blob/master/config/postal.defaults.yml for all possible configuration options
Be sure to do some port mappings to allow your SMTP server and HTTP(s) server to be accessed. I suggest simply port mapping the SMTP server and using [jwilder/nginx-proxy](https://github.com/jwilder/nginx-proxy) to proxy the HTTP server.