@jimcarroll @molly0xfff #GhostBlog has separate configuration for transaction email and bulk email.
If you want to send out both through Mailgun, you can send the transactional emails out through the Mailgun SMTP interface.
You can send (only) transaction emails out through SES also via SMTP.
The Ghost project is open supporting other providers for bulk email, but first an "adapter" needs to be created that abstracts the details for each provider.
Ghost also uses Mailgun for analytics.