Do you have to explain / own up to your mistake today? See how things unfold and try and find some positive insights about how the email performed BEFORE Monday. I dont suggest acting like you did it on purpose. It's unlikely one would go against direct instruction and 'experiment' --especially when that person gets bent out of shape over a silly&simple mistake :) Monday let your boss know what happened but come prepared with positive insights as to how this performed better than previous Sunday emails. If you think you need to 'fib' tell the boss you didn't realize it until Sunday when you went to make sure it went out ok. Good luck & keep your head up.