If this is in you app.yml Discourse will create a file in your app container at var/www/discourse/config/multisite.yml the content will be the same. I know it is confusing when you are following the guide. But it is best way for multisite to declare secondsite or my be third in app.yml. I am using multisite and a self hosted postfix, dovecot setup for sending and receiving emails. And at this moment it is working like a charm. I just configured postfix with virtual mailboxes. Postfix retain reply key as well. It is a little bit more work but in my view is better solution.