Just had a great conversation with SendGrid – we’re already using their API for sending in-app messanging transactionally, at work. The API is easy to use, and you have great reporting on email deliverability through the web interface. Just chatted with two representatives at Women2.0′s Startup Weekend event- they’re sponsoring some prizes.
I’m also a fan of MailChimp- and have setup various clients on them. The Flash/UI interface is really great, and the reporting on the various email campaigns is also easy. They, like SendGrid, have a very communicative and readily available customer service – for engineering and design.