The welcome email is a crucial part of a customer's first interaction with a product, providing an opportunity to offer useful information that helps them explore the product. Effective welcome emails from popular products like Slack, Harvest, Basecamp, and FreeAgent highlight the importance of friendly copy, clear call-to-actions, important account details, resources for getting started, reply-To addresses for customer support, and links or phone numbers for contacting support. A well-designed welcome email can help improve engagement rates by providing a positive first impression and setting customers up for success with the product.