Listmonk

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.