Plugin dev with Sendinblue mailer integration
Looks like you're missing the proper namespace.

Should be:

$mailin = new \Sendinblue\Mailin('https://api.sendinblue.com/v2.0', 'dqsdsapicode', 5000);


And here also is the other syntax for a global call in the controller.
Controller beginning would be:

&lt;?php
namespace Craft;

use Sendinblue\Mailin;

class FooController extends BaseController


Then class call will be:

$mailin = new Mailin('https://api.sendinblue.com/v2.0', 'dqsdsapicode', 5000);