Mailgun is probably the service that focuses most on the transactional emails. In fact that is there entire business. They also focus on being developer friendly, as that is their primary audience. They allow for 10,000 emails to be sent each month for free, which is a great way to get started. I have used Mailgun a couple of times, and found it to be a good service. They offer API based sending as well as an SMTP service. However, once your needs increase, they do seem to be the most expensive of the group.