Ask HN: Event or Data Driven Email Service

I'm trying to find a way to send out nicely formatted email to a single user based on an event or data change. For example, send a reminder when a user's trial period is 10 days from expiration. I could roll my own, but right now I believe my time is better spent on functionality for my application. Down the road I would like to automate the process of sending out such emails, but for right now we have very low traffic, so I can check for events and send the messages myself. I've looked into options such as MailChimp and Campaign Monitor, but those seem to be tailored for newsletters or sending to lists. I am hoping there is a service where I can create a template, fill in some user specific data, and send the email manually (an API for later automation would be great). Any suggestions would be greatly appreciated.