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Google's Mail API for App Engine seems to still be available. I think they don't really want you to use it, but there it is.

Postmark is pretty good as well :)

Mailjet, mailgun, sparkpost and a bunch of others.

Mailjet / Mailgun are one and the same service and since the acquisition, I haven't heard of anyone still happy with them. But yes good point, Mailjet is another one.

Sparkpost to my knowledge is built on SES.

Sparkpost roll their own MTA’s on AWS, they’re not sending via SES.