Amazon SES is awesome, said MailChimp 1 year ago.
Developers do need transactional email API (1-to-1 email, as compared to MailChimp's 1-to-many service) for sending bills, receipts, etc. And so MailChimp integrated Amazon SES into their service, providing many of the features that MailChimp already provides.
Then 1 year later.. Mailchimp has a Mandrill :)
Mandrill is not based on Amazon SES. It is quite obviously a rival, as they deprecate MailChimp STS that integrates Amazon SES.
There are also other transactional email API providers like MailGun, Sendgrid, ...