See Why do I have to set up Mailgun for newsletters? …
Did you know Mailgun doesn’t have free accounts anymore??
Mailgun recently updated their pricing, including the removal of a free tier. However, under their new pricing, invoices are not sent if they total less than $0.50 - which is still more than 600 emails. For most people who are getting started Mailgun remains free, and subsequent rates are extremely generous. It would cost far, far more (money as well as time) to run your own bulk mail delivery infrastructure, or use most other providers.