Transactional Email
Developer-friendly email APIs.
The API is the easy part. What you're actually renting is sender reputation — shared IP pools, bounce handling, and how fast a human answers when your mail starts landing in spam. Decide early whether you need email alone or SMS and voice on the same bill, because that splits the field cleanly.
Editor's pick
Postmark, if deliverability is the whole reason you're shopping. Keeping bulk and transactional streams apart is a discipline, not a feature toggle.