I manage a couple of Postmark accounts on behalf of clients. It’s a solid service, just like Mailgun and all the rest.
Their free tier allows just 100 emails per month though, which is barely enough for testing purposes. And if I have to pay then I’d rather stay with the Mailgun that I’ve used for years.
For my own use, I’m currently using AWS SES for both transactional and bulk emails, using FreeCodeCamp’s Mail for Good as a front-end to manage my newsletters and bulk email campaigns. Yet to do any integration with Ghost.