listmonk is a free, open-source, self-hosted newsletter and mailing list manager that also sends transactional email. It offers single and double opt-in lists, subscriber segmentation with SQL expressions, built-in campaign analytics, a transactional API, queues for multiple SMTP servers with rate limiting, and templates with a drag-and-drop builder, Markdown or HTML. listmonk is a single Go program that uses PostgreSQL; you install it as a binary or with Docker Compose.
listmonk is licensed under the AGPLv3 and was created by Kailash Nadh, chief technology officer of the Indian broker Zerodha. The project therefore originates in India and lives on GitHub. There is no official hosted service; the website does list third-party hosting providers, with which the project is not affiliated. Because you run listmonk yourself, subscriber data and the database stay on infrastructure of your choice, for example in the EU. You also decide which SMTP service the email goes through.