This document provides guidance for product managers on using email notifications, alerts, and other transactional messages from apps to engage users and grow the app. It discusses key use cases for product emails, how to measure their impact, and the technical challenges of reliably sending large volumes of emails from an app. Specific topics covered include common types of emails like onboarding, reminders, and security alerts; identifying important "make-or-break" moments where emails can significantly impact user behavior; and how delivering emails at scale from an app is more complex than a simple "send" button due to challenges like deliverability, spam filtering, and transactional email infrastructure requirements.