Hey,
I have encountered error in InjectionResponseParser class.
Uncaught Error: Cannot use object of type stdClass as array in vendor/socketlabs/emaildelivery/InjectionApi/src/Core/InjectionResponseParser.php:27
I think ErrorCode should be taken by array because $injectionResponse comes from json_decode function in 18th line.
$errorCode = $injectionResponse->MessageResults[0]['ErrorCode'];