Polymorphic routes, :as => "polymorphic_name"

The polymorphic_path helper is designed to build route names from the class name of the object passed to it with. Therefore if you pass it a message object then that's what it will use - it doesn't look at the application's routes, it just calls the route helper name that it builds from the arguments. You may want to look at refactoring your routes to something like this:

  # config/routes.rb
  resources :users do
    resources :messages, :except => :index do
      collection do
        get  :received
        get  :sent
        post :send
      end
      member do
        post :send
      end
    end
  end

Then in MessagesController you can implement the custom actions for received messages, sent messages and sending a message directly or after saving a draft. This way you can map the standard REST actions like new, create, edit, update to dealing with drafts. The destroy action would then either delete an unsent draft message or delete the link record from the appropriate table depending on whether it's a sent or received message.