Your question is Power-Analysis for Email Message Test. 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 a customer email program and want to test a new message to customers. The goal is to see whether the revised email improves engagement without hurting downstream behavior.
How would you design a simple experiment to test the new email message to customers? Walk through how you would define success, choose the randomization unit, and decide whether the result is strong enough to ship.