In Ghost we cannot use any other smtp services for sending newsletters other than Mailgun.
In my scenario, I am using AWS for hosting (because I got AwS credits for partners) and I have 200,000 per second sending rate limit. If you are using AWS EC2 for hosting, you can send 62,000 emails per month for free with AWS SES
I would like to suggest to Ghost to integrate AWS SES for sending newsletters.