Plunk is a open-source email platform for transactional and marketing emails with automation. It handles open source, automations, transactional, and analytics, and it's best suited for developers wanting open-source transactional email. Launched in 2022, it's a relatively recent entrant that's been gaining traction.

The core product is entirely free. Since it's open-source, you can self-host for free with no user limits. It's designed for small teams, striking a balance between simplicity and collaboration features.