Postmark is an e-mail delivery service.
The Postmark API uses a REST interface, via HTTP.
In order to use Postmark, users must sign up with Postmark to obtain credentials. (Postmark is not free; as of 2015-11, it costs about 1/8 of a cent to send an email to a single recipient.)
This interface is incredibly thin; adding new calls is super-simple. In fact, the abstract-o-matic inside me wants to simplify all of this to a simple declarative api-specification-language.
For more information on the API, see http://developer.postmarkapp.com/
(send-single-email server-api-key #:From from #:To to #:Body body [ #:Subject subject]) → jsexpr? server-api-key : bytes? from : string? to : string? body : string? subject : (or/c string? #f) = #f
(deliverystats server-api-key) → jsexpr?
server-api-key : bytes?