Mailgun has a RESTful API with libraries and code examples in curl, Ruby, Python, PHP, Java, C#, Go.

Mailgun has a simple pricing scheme where you pay for what you use without punitive overages. Every account has access to the same set of features once you add a credit card to your account.

Good documentation for SMTP. Changing an old Django/SMTP/Mandrill configuration (broken by Mailchimp price plan changes) to Django/SMTP/Mailgun was a breeze.

For customers that are sending high volumes of messages, Mailgun offers a managed service level, which pairs you with an e-mail expert to help you get your messages delivered.

Mailgun is included and configured by default in Meteor.js. So for projects which are built using Meteor, it's very easy to integrate Mailgun.