toolmantim uses Campaign Monitor
Campaign Monitor handles the subscriber lists as well as all the transactional emails (the signup welcome email, and the course purchase thank you email). Emails are tracked and edited directly in Campaign Monitor without needing to modify the Rails application, and allow non-tech admins to check email deliveries and reattempt emails.
BrightMachine uses Campaign Monitor
Provided to our clients to help them gather email lists and send great emails.