mailer.rb

  def new_idea_notification(idea, user)
    @idea = idea
    @user = user
    headers("X-Foo" => "foo")
    headers["X-Fie"] = "fie"
    email = mail(:to      => user.email,
                 :subject => "New Idea: #{@idea.title}",
                 "X-Bar"  => "foo")
    email["X-what"] = "why"
  end