At times you do can get a instant message shortly after transmitting the return. Other times as indicated by @patrick you receive an error message.
At the June 15 self-employed deadline I had one couple who authorized Express NOA but CRA did not recognize the authorization (I have verified that the transmission files had the correct code 1). As of June 20 RAC shows the returns as “being processed”, yet there was nothing unusual about them to my mind. For another couple the returns were reported as assessed instantly but the “tomorrow” when the NOAs will be available has been moving forward one day every 24 hours for 5 days so far! RAC says NOAs will be available to view June 27. Some Express service!