I'm curious more along the lines of personalized emails to users of a service. As far as I know, MailChimp is more about blasting one message to lots of people. Unless I'm mistaken :)
Oh, okay. In that case you want to look at transactional email services. MailChimp offers that as well with https://mandrill.com, but there are a ton of other offerings as well like http://sendgrid.com.