Action Mailbox: The Missing Manual

action_mailbox.md

This is all you really need to know in order to make Action Mailbox work in development.

1. Fire up ngrok http 3000 and make note of your subdomain for steps 3 and 8.
2. Create a Mailgun account because they offer sandbox addresses; grab your domain from the Dashboard.
3. Go into Receiving and create a catch-all route pointing to: https://XXX.ngrok.io/rails/action_mailbox/mailgun/inbound_emails/mime
4. Add your Mailgun API key to your credentials:

action_mailbox:
  mailgun_api_key: API KEY HERE

5. rails action_mailbox:install
6. rake db:migrate
7. rails generate mailbox incoming
8. Add the following to your config/environments/development.rb

  config.action_mailbox.ingress = :mailgun
  config.hosts << "XXX.ngrok.io"

9. Make sure your config/sidekiq.yml is monitoring the action_mailbox_routing queue.
10. Configure your application_mailbox.rb with whatever routing you need; example below.
11. Configure your incoming_mailbox.rb as you wish; example below.

You should now be able to send an email to test@MAILGUN-SANDBOX-DOMAIN.

application_mailbox.rb

class ApplicationMailbox < ActionMailbox::Base
  # routing /^info@/i => :incoming
  routing all: :incoming
end

incoming_mailbox.rb

class IncomingMailbox < ApplicationMailbox
  # before_processing
  # around_processing
  # after_processing

  # before_processing do
  #   unless User.exist?(email: mail.from)
  #     bounce_with Mailer.user_not_found(mail)
  #   end
  # end

  def process
    # puts mail.methods - Object.methods
    # mail.content, mail.subject, mail.has_attachments?
    puts mail.inspect
  end
end

One small change needed here..
You shouldn't call #deliver_later on the ActionMailer message when passing it into bounce_with because #deliver_later gets called on the message internally.

Here's the source from ActionMailbox::Base#bounce_with

# Enqueues the given +message+ for delivery and changes the inbound email's status to +:bounced+.
    def bounce_with(message)
      inbound_email.bounced!
      message.deliver_later
    end

@stuarthub thanks so much! I'll update right now.

Thank you very much, I will take everyone's suggestions seriously