Rails 6 ActionMailbox with Mailgun returning 404 - ruby

Question

I am developing an in-house expenses application on Rails 6 with ActionMailbox to replace Excel spreadsheets. As many of our receipts are now in email form (for example airline tickets), the idea is that users will be able to simply forward a receipt to the application and it will automatically be associated with the expense entry.
I am using ActionMailbox with Mailgun as the email receiver. As suggested by the Gorails Pro tutorial I have exposed my app using localtunnel to the general Internet. I have used Mailgun's facility to send a test email to my application.
My post address is:
https://xxxxxxxx.localtunnel.me/rails/action_mailbox/mailgun/inbound_emails/mime
However, I have run into an issue where the incoming email from Mailgun is not being processed correctly but is being returned with a 404 error. The Rails log shows the message being received as a POST. The last two entries in the log are:
2019-10-15T07:50:07.646Z 10260 TID-gn609ivg8 INFO: Filter chain halted as :ensure_configured rendered or redirected
2019-10-15T07:50:07.646Z 10260 TID-gn609ivg8 INFO: Completed 404 Not Found in 0ms (ActiveRecord: 0.0ms | Allocations: 144)
My configuration is:
config/routes.rb
Rails.application.routes.draw do
devise_for :users
resources :categories
resources :expense_claims do
get 'export_excel', on: :member
post 'barclay_csv_import', on: :collection
end
resources :expense_entries
root 'expense_claims#index'
# Enable the sidekiq console.
require 'sidekiq/web'
mount Sidekiq::Web => '/sidekiq'
end
config/application.rb
module Expenses
class Application < Rails::Application
# Initialize configuration defaults for originally generated Rails version.
config.load_defaults 5.2
# Settings in config/environments/* take precedence over those specified here.
# Application configuration can go into files in config/initializers
# -- all .rb files in that directory are automatically loaded after loading
# the framework and any gems in your application.
# Set the ActionMailbox ingress here for now.
config.action_mailbox.ingress = :mail_gun
end
end
config/environments/development.rb
Rails.application.configure do
[... lot of stuff removed as not relevant]
# Settings specified here will take precedence over those in config/application.rb.
config.action_mailer.default_url_options = { host: '0.0.0.0', port: 3000 }
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = { address: '0.0.0.0', port: 1025 }
# Set the active job queue adapter to Sidekiq/Redis
# config.active_job.queue_adapter = :sidekiq
# Alternatively, when debugging, you can set to in-line (or :async)
config.active_job.queue_adapter = :inline
# Don't care if the mailer can't send.
config.action_mailer.raise_delivery_errors = false
config.action_mailer.perform_caching = false
# Set so we can test Devise self registration
config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }
# Allow traffic from localtunnel
config.hosts << 'xxxxxx.localtunnel.me'
end
app/mailboxes/application_mailbox.rb
class ApplicationMailbox < ActionMailbox::Base
routing :all => :receipt
end
app/mailboxes/receipt_mailbox.rb
class ReceiptMailbox < ApplicationMailbox
# mail => Mail object
# inbound_email => ActionMailboxEmail record
def process
end
end

Answer 1

Well, turns out it was the most stupid of mistakes. In config/application.rb the final configuration was wrong. I needed to replace :mail_gun with :mailgun:
# Set the ActionMailbox ingress here for now.
config.action_mailbox.ingress = :mailgun
Unfortunately, the Rails error message was not very helpful here.
To give a little more info, ActionMailbox has a number of different routes defined for different email processors. Therefore, the URL to which the email processor posts the incoming email defines the controller to use. On receiving an email, Rails checks that the URL to which the email has been posted matches the ingress class that is set in the config. If not, you get the error I saw.