I moved my side project from Mandrill to SendGrid immmediately after the “absorbtion” (Mandrill/MailChimp policy changes). But then I had to scramble to move again after an incident where SendGrid apparently felt it was OK to delay sending transactional emails by a day or two, affecting many of their paying customers. [1]

Currently on Postmark, and have found them to be reliable, transparent about the very rare problem on their end, and helpful with problems on my end.

Following Mandrill’s policy changes, I started maintaining a package that simplifies moving Django projects between a number of full-featured transactional ESPs. Even if you’re not on Django, you may find the list of ESPs it supports helpful. [2]

[1]: https://news.ycombinator.com/item?id=12142728

[2]: https://anymail.readthedocs.io/