Hmm. Yes, first you’ll need to configure the mailgun the mail receiver to have some means for delivering mail, as it doesn’t know about the credentials or transport mechanism in app.yml. I think you’ll need to add a more complete configuration as hinted in the next section about mounting volumes, the details of which are beyond the scope of this document.
The simple solution for “how do I deal with postmaster and admin emails” is to create a group for each and add whoever you want to get those emails to that group and they can deal with them as group messages.