You should use docker's ambassador containers and avoid hard linking between containers to ensure service portability. This will easily allow you to change your settings if you need to talk to a different or additional postfix container. You simply restart the postfix-ambassador container that your mail box is connected to. In the case of mail processing where you might want to add various other processing stages, e.g. SpamAssassin, ClamAV, etc., this approach will help in creating a chained process.
This pattern supports the moving of your postfix server to a different docker host.
Here is a blog post describing the pattern - http://fosiki.com/blog/2013/12/05/docker-container-network-portability/