Ask HN: What do you suggest for action-triggered email?

We have created a social network for startups and while the in-platform notification system works great in our private beta, I believe that transactional email notifications (those triggered by user actions) are critical to user retention.

We intend to use Mailchimp for newsletters, but what services can you recommend for transaction (automated following user actions) email?

Sendgrid seems to be the popular choice, but Mandrill is another option– and as its a Mailchimp product, perhaps this could work for us?

We need something that's easily implemented and scalable.