Key metrics to measure during your tests include: - Open Rates: The percentage of recipients who open your email. - Click-Through Rates (CTR): The percentage of recipients who click on a link within your email. - Conversion Rates: The percentage of recipients who complete a desired action, such as making a purchase. - Bounce Rates: The percentage of emails that could not be delivered to the recipient's inbox. - Unsubscribe Rates: The percentage of recipients who opt out of your email list after receiving your email.