A test is valid when you change only one element, send it to a similar audience, and collect enough data to see a clear difference. Smaller lists can still run meaningful tests. Just keep them simple and allow more time for results.

Yes. Most modern tools remain reliable even with privacy updates. They use several signals such as clicks, conversions, and inbox activity to help you understand real engagement.

A weekly review works well for most brands. It helps you track trends without getting lost in daily changes. High-volume senders may benefit from daily checks to catch issues early.