Test email sending with Mailgun?

I don’t see the ability to test email sending from the Labs panel in the Ghost admin application. How can I test if email sending is working?

Have you configured Mailgun in your config.production.json and the member’s section of the Ghost?

Yes, I added the API key and domain to the Email newsletter settings.

How do I configure config.production.json? Is that the same thing as “Config Vars” on the Heroku settings page? If so, that has already been set appropriately.

EDIT: I did not see this file in the Ghost root directory I cloned from Heroku.

You cannot see config file in repo because this file is dynamically generated with your secrets like api keys and all. You can change these values in Heroku config vars. There you will find keys related to mailgun and all other secrets used in your Ghost blog.
To know how to change config vars in Heroku read this article - How to create, update, and delete config vars in the Heroku app?

I will suggest you to change config vars using Heroku settings page. Read in article about changing config vars using Heroku settings.

This is probably not working for me because I am using Gatsby and Netlify to serve the site. Thanks for your advice.

Are you able to locate your config.prodcution.json?
Try to locate your API keys and all in netlify Build environment variables

I figured it out. It wasn’t working because sandbox domains in Mailgun may only send emails to verified recipients.