How to test Mailer on Sinatra post request?

I have Sinatra app which sends email on post request:

post '/test_mailer' do
  Pony.mail(
    to: 'me@mine.com.au',
    from: 'me@mine.com.au',
    subject: 'Howdy!',
    body: erb(:body) )
end

So I want to test this behaviour using:

require 'spec_helper'

describe 'App' do
  before(:each) do
    Pony.stub!(:deliver)
  end

  it "sends mail" do
    Pony.should_receive(:mail) do |mail|
      mail.to.should == [ 'joe@example.com' ]
      mail.from.should == [ 'sender@example.com' ]
      mail.subject.should == 'hi'
      mail.body.should == 'Hello, Joe.'
    end

    Pony.mail(to: 'joe@example.com', from: 'sender@example.com', subject: 'hi', body: 'Hello, Joe.')
  end

  it 'test_mailer' do
    Pony.should_receive(:mail) do |mail|
        mail.to.should == ['me@mine.com.au']
    end
    post '/test_mailer'
  end

end

This is my spec_helper

require File.join(File.dirname(__FILE__), '..', 'app.rb')

require 'sinatra'
require 'rack/test'

# setup test environment
set :environment, :test
set :run, false
set :raise_errors, true
set :logging, false

def app
  Sinatra::Application
end

RSpec.configure do |config|
  config.include Rack::Test::Methods
end

But I receive error:

mailer(master)» rspec spec/app_spec.rb
.F

Failures:

  1) App test_mailer
     Failure/Error: Pony.should_receive(:mail) do |mail|
       (Pony).deliver(any args)
           expected: 1 time
           received: 0 times
     # ./spec/app_spec.rb:20:in `block (2 levels) in <top (required)>'

Finished in 0.02542 seconds
2 examples, 1 failure

Failed examples:

rspec ./spec/app_spec.rb:19 # App test_mailer

So, how should I properly test that post '/test_mailer' request?

Maybe I'm being dumb, but it looks quite obvious - there's nothing in your production code which calls a deliver method. Shouldn't you be verifying Pony.should_receive(:mail) ?

Update: I see Pony has a private class method called deliver, but you're stubbing that, so it will never be called.