Transactional email is a tough thing to properly evaluate. Before joining Wildbit and spending a lot of time working Postmark, I was incredibly ignorant of just how complex of a topic it can be. I put together a guide to help objectively evaluate transactional email service providers. It’s on the Postmark site, but I’ve done my best to keep it impartial.
Really, all of our transactional email guides are written to be impartial and hopefully useful regardless of which email provider you use. I’ve personally written the vast majority of these guides, and we’ve really tried to make them universally helpful to increase awareness and education around transactional emails independent of providers. I hope it helps.