Comment by jrblo
I've found Mailgun's services to be pretty good. Admittedly I haven't tried many of the alternatives in recent years, but we use Mailgun to send a few hundred thousand transactional messages per month at my day job and they have been good. For my personal side projects, their free plan has been sufficient enough that I've never paid for usage.
Mailgun is great. I keep coming back to it after trying many alternatives.