A number of the bigger transactional mail providers actually run on top of AWS SES including SendGrid which is recommended by Google. AWS SES looks like a good option.
What do you like about Mandril and what features would you love to see added?
Mandrill is great. They already have handlebars support which is good. But handlebars itself lacks some important things like checking equality between strings etc. I think a better templating engine seems like a nice idea