Measuring the impact of testimonials in your email campaigns can be done through: 1. A/B Testing: Compare emails with and without testimonials to see which performs better. 2. Engagement Metrics: Track open rates, click-through rates, and conversion rates. 3. Customer Feedback: Ask new customers if testimonials influenced their decision to purchase. 4. Sales Data: Analyze sales data to see if there is an uptick in purchases following campaigns that include testimonials.