Your question is Incrementality Test for Email Campaign. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You work on an operations team that wants to launch a new email campaign and measure whether it actually drives incremental behavior, not just activity that would have happened anyway. You need a clean control cohort so leadership can trust the result before scaling the campaign.
How would you set up the holdout or control cohort to measure incrementality of the campaign, and how would you decide whether the campaign is worth shipping?