Acelle: From this page, you can configure your Acelle settings. You can check the connection status, set the API endpoint, and enter the API key. Once the setup is complete, click "Save" to update your settings successfully.

Acelle Mail is a self-hosted, open-source, full-featured and easy-to-use Email Marketing. 

Here is the demo link below from the acelle API key. 

Link: https://demo.acellemail.com/admin/docs/api/v1