Ghost is really amazing project, except for the fact it forces us to use Mailgun.
It doesn’t really make sense to force self-hosting applications to use third-party email servers.
Forget Newsletters, at least leave signup and signing alone.
You can easily set these transactional emails up with any SMTP.
Here’s an example with Amazon SES, for example, but you can literally use any SMTP connection.
Amazon is not totally free either, and well it’s also 3rd party isn’t it?
You can always set up your own SMTP server to send transactional emails. As I mentioned, any SMTP connection is sufficient.
Also, Ghost is open source. So, if the existing options aren’t suitable for you, you can always change things as you want.