I would be glad to take another look at Mailgun. Can you easily do segmenting (i.e. Only send this message to people where Job Title == 'CEO')? I think that may have been the dealbreaker when I last looked. Exposing the user's mail agent in analytics would also be nice.

And although I wish I could say all I need is a kickass API, I actually will need a GUI suitable for marketing folks too. Happy to followup privately since my requirements may be slightly unique.