Postmark is offering this as well. It's been in private beta for a number of months but is solid. It's been in production on our other app, Beanstalk, as well o

Postmark is offering this as well. It's been in private beta for a number of months but is solid. It's been in production on our other app, Beanstalk, as well our customers apps including the issue tracker DoneDone (http://www.getdonedone.com http://www.getdonedone.com). Beta is closed now until we open the feature to the public, which is planned to be very soon.