Amazon SES is great, because you pay per email and the rate is incredibly cheap. Mailgun is very expensive though, and the problem is they have a very limited free tier, and then you must jump up to a $35/year package that gives 50,000 emails - this is simply far too much for many projects early on in their rollout.

I prefer something like Brevo, which has smaller jumps per tier or even something like MXRoute for $49 per year (limit of 300 emails/hour)

Edit: Completely forgot about ZeptoMail by Zoho - incredibly good value service.