unless Rails.env.production?
  class ActionMailer::Base
    def render_message_with_path_protection(method_name, body)
      body = render_message_without_path_protection(method_name, body)
      if body =~ %r{((href|src)=['"]/.*)}
        raise "Use absolute path in mail urls! #{$1}"
      end
      body
    end

    alias_method_chain :render_message, :path_protection
  end
end