Mailpit

A compact email testing tool written in Go that runs as an SMTP server and shows captured messages in a modern web interface. Beyond viewing HTML, text and raw MIME sources, it offers an HTML compatibility checker, a link checker, a spam score through SpamAssassin, screenshot generation, message tagging and a REST API for automated assertions.

Teams use it to verify signup and notification flows during development, and a Chaos mode can inject SMTP errors to test how an application handles delivery failures.

It runs as a single binary with no external dependencies to install, and every test message stays on your own network, where you can wipe the mailbox between runs without touching a shared sandbox.