TL;DR if you're building an email app or service, you have just likely found the answer to speeding up your development significantly.
I've been using it for 6 months now (albeit in dev). They put out frequent updates, and it's especially easy to get running in several ways (all in docs but I use Digital Ocean and it's less than 5 minutes to get set up).
I have yet to get my app verified by Google which I think is the biggest hurdle ahead of me, but having Email Engine will only help me clear that hurdle.
Finding this project sped up my (solo) development by at least 12 months. I will forever be grateful that I found the founder's/lead engineer's response to help a stranger first and _then_ pitch the business.
Also, they're very engaging on Github. And even when I'm totally in the wrong, they're kind when they point it out.
