That seems doable with something like alphamail, but I've yet to see a service that targets that specific need.
Like Amazon SES on steroids.
P.S. Not affiliate with amail, just an enthusiastic user. We are using that at CoffeeStrap and we just saved a lot of time in development.
I suggest we replace Email with a superior solution: http://retroshare.sourceforge.net/
My only concern is the deliverability, but I'm intrigued enough to give it a try seeing as how there's a free tier.
One feature I really need though would be support for internationalization, since creating a separate project for each language is quite annoying (and the reason why I'm not using MixPanel Engage.)
Do you have plans for supporting that? If you could integrate with OneSky that would be even better! :)