Unless I’m misquoting, the entire point of Ghost requiring Mailgun for newsletters is because you shouldn’t (or can’t, or will have a really bad time if you) use a transactional sending client for bulk email. Ghost uses the Mailgun API to send newsletters, and the mail client configured in your config for transactional emails. This also allows for more personalized newsletters (e.g. Hey {first_name},)
Docs: Why do I have to set up Mailgun for newsletters?
@GregFdz: As @mjw and @Cathy_Sarisky have mentioned, if you’re on the free tier, and you send out 1 newsletter a week to 2,000 people, that’s 8,000 emails being sent for $8. If that’s too much, it would probably be better to use a different ESP (as in, don’t use Ghost’s built-in newsletter functionality, but use the other parts of it). If you really want Ghost to support more bulk email providers than Mailgun, there’s an open call for help: Support sending bulk email from other providers - #5 by Hannah