So you can’t get @Mailgun to accept incoming emails and forward them to your heroku app? It took me a couple of hours to figure this out. I was trying to send emails to my app through Mailgun to create new bookmark entries in my Davemarks app. Everything seemed to work right. I double checked my incoming controller, my setup mail initializer (config/initializers/setup_mail.rb), my enviroment variables (configured using Figaro) and my production environment (config/environments/production.rb). All ok.
The error resulted from me not verifying my account through their sms system. Always make sure the email for the account is verified to avoid annoying bugs like the 550 5.7.1 Relaying denied.