Plunk

Plunk handles both transactional emails (welcome emails, order confirmations) and marketing emails (newsletters, drip campaigns) from one open source platform you control. Self-host it, connect your SMTP or AWS SES, and you've got email infrastructure without paying per-email fees to Mailchimp or SendGrid.

The dashboard is clean. Manage contacts, build emails with a visual editor, set up automations, track opens and clicks. It's what you'd expect from a modern email platform, minus the monthly bill.

AGPL-3.0, TypeScript. They also offer a hosted version.

The catch: AGPL license means if you modify Plunk and offer it as a service, you must open source your changes. The project is relatively small compared to alternatives: fewer integrations, less battle-testing at scale. And email deliverability is hard. Self-hosting email means managing SPF, DKIM, DMARC, IP reputation, and bounce handling. Plunk makes the app layer easy but can't solve the deliverability puzzle for you.