Enabling Rails 4.1 mail previews in staging (or any other env besides development)

staging.rb

# put this in your staging.rb file. Obviously you'll need more config than this it's just an example.

Rails.application.configure do
  config.action_mailer.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/spec/mailers/previews" : nil
  config.action_mailer.show_previews = true
end

# Without the following would display "For security purposes, this information is only available to local requests"
class ::Rails::MailersController
  include Rails.application.routes.url_helpers

  def local_request?
    true
  end
end

# assuming you've configured your mail previews in your test folder this will work fine