Upyo is a cross-runtime, cross-provider email library for JavaScript and TypeScript that abstracts email delivery across Node.js, Deno, Bun, and edge runtimes using web standard APIs. Unlike Nodemailer which relies on Node.js-specific modules, Upyo supports multiple providers (SMTP, Resend, SendGrid, SES, etc.) through a unified Transport interface, allowing applications to switch providers and runtimes without code changes.