You deal with this by adding .and_call_original
to your method chain as follows:
UserMailer.should_receive(:new_order).with(order).and_call_original
as documented in https://www.relishapp.com/rspec/rspec-mocks/v/2-13/docs/message-expectations/calling-the-original-method and discussed in Is there a less intrusive alternative to Rspec's `should_receive`?