The Postmark App for Stripe is a new integration that allows users to send emails to their customers without leaving the Stripe platform. This app brings email sending functionality into Stripe, making it easy to build relationships with customers, reduce churn, and grow business through targeted communication. Users can choose between HTML and plain text templates, track customer engagement, and view activity stats within Stripe. The Postmark App is now available in the Stripe App Marketplace for easy installation.