Not only is transactional email a great referral channel (just look at what Dropbox did), it's also ripe for optimization. Engagement emails are used by almost

Not only is transactional email a great referral channel (just look at what Dropbox did), it's also ripe for optimization. Engagement emails are used by almost every kind of app to facilitate user actions, but by being trapped in source code, the people who care about user experience and metrics can't take action to improve them.