The document discusses building applications for Yahoo! OpenMail. It provides an overview of application capabilities, the development process, and code examples. Specifically: - Applications can access users' email content and calendars, and trigger new email messages. Storage space of up to 5MB is available. - Developers set up accounts, enable OpenMail access, and use web or command line tools to code, test, and deploy applications. - A basic "Hello World" example demonstrates configuring an app, launching a view on click, and composing a test email via the OpenMail API.