Listmonk

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Listmonk is a free and open-source email marketing tool that allows you to send newsletters, promotional emails, and transactional emails. It's a self-hosted newsletter and mailing list manager that's fast, feature-rich, and packed into a single binary.

Listmonk has these features:

Supports powerful and dynamic email templates
Allows you to create and manage massive mailing lists, subscribers, and campaigns
Enables you to create customized campaigns and manage templates and media that can be reused in templates
Uses a PostgreSQL (⩾ 12) database as its data store
The back-end is written in Go, and the front-end is Vue with Buefy for UI
Licensed under AGPLv3

You don't have to pay for subscriptions or worry about hidden fees with Listmonk. You only pay for hosting the service.