Assuming you completely randomize which half goes into each split, it wont vary significantly over time.
Obviously some segments are going to open/click/spam at higher rates, but that's fine and controllable with a customer rating. So when you split your users, ensure an even makeup of customers, and your tests will be fine.
Obviously some segments are going to open/click/spam at higher rates, but that's fine and controllable with a customer rating. So when you split your users, ensure an even makeup of customers, and your tests will be fine.