Courier is a development platform for notifications that offers an in-app notification inbox SDK for web and mobile apps. It provides a ready-made, customizable UI to simplify the process of crafting an intuitive notification interface, as well as managing back-end processes such as server setup and database handling. Courier's APIs facilitate seamless communication between front-end and back-end systems, making it easy to implement personalized notifications based on user preferences. The platform also coordinates read status across multiple platforms like web, Android, and iOS, ensuring a consistent notification experience for users. With its React SDK, Android SDK, and other tools, Courier enables developers to build versatile in-app notifications for both web and mobile applications, overcoming common challenges such as designing and maintaining an effective notification system.