emails_controller.rb

class EmailsController < ApplicationController

  ##
  # Receive the AWS post, validate the token and pass to our model method
  def incoming
    if params[:token] && (params[:token] == 'test_token')
      @email = Email.process_incoming_email(params[:message_id])
      if @email.valid?
        render text: 'email created', status: :created 
      else
        render text: @email.errors.full_messages, status: :unprocessable_entity
      end
    else
      render text: 'unauthorized', status: :unauthorized
    end    
  end


end

emails_controller_spec.rb

  context 'receive a post from Lambda' do
    it 'should be receive a message from email via Lambda' do
      expect { post(:incoming, token: 'test_token', message_id: 'test') }.to change { Email.count }.by(1)
    end

    it 'should be be unauthorized without the lamda token' do
      response = post :incoming, message_id: 'test'
      expect(response.status).to eq(401)
    end
  end