When a prospect opens a cold email twice but doesn't reply, the first thing I would test is the call-to-action friction, especially how easy it is to respond in one step. Open-rate behavior signals attention, but repeat opens without replies often indicate interest without conversion clarity, similar to tracking query engagement without clicks. I would instrument reply paths like a funnel, checking whether the email's CTA asks too much effort, lacks a clear prompt, or forces uncertainty in next steps.