This kind of question is like asking a baseball shortstop a hypothetical: What do you do if the ball is hit to you.

Anyone who tries to answer it definitively is probably not going to give a good answer. You need to get context out of the situation before you can answer it well. Just like you wouldn't throw the ball to first base if you could try to turn a double play, you wouldn't expect similar open rates between a list made up of young tech savvy users and a list made up of senior citizens. You wouldn't expect similar open rates between a list that you have to opt-in to on a preferences page and a list that you are automatically signed up to when you register for a product.