Your API is RESTful (http request/response) but you do not provide raw HTTP request examples. You only provide examples which use cURL, C# classes, etc., which has the effect of concealing your actual API. IMO this is ridiculous that you don't actually document the API itself, which consists of HTTP/HTTPS requests and responses.

You don't need to document the responses. But I definitely need you to document example HTTP / HTTPS requests.

Your documentation says nothing about List-Unsubscribe, only about common unsubscribe links. You should update your documentation to reflect information about List-Unsubscribe.

It would be great if you guys would add Google Apps Script to your how-to's and code samples. Google has reduced the amount of emails many clients are able to send from their Mail API resulting in a flurry of conversation in the Google forums. One user has posted a workaround with some Google Apps Script Code that utilizes MailGun, which is how I found you.

This seems like a great little marketing niche/opportunity for you guys.

See 4th post in this thread: http://stackoverflow.com/questions/21444588/has-google-lowered-the-email-limits-for-google-apps-script

Your entire API for PHP demands that you can use Composer. However PHP is being used on a number of different platforms, amongst others, Windows, where the use of Composer is not that normal, so it would be nice, if you just had an example of a simple implementation using simply the basic PHP functionality.