The class Swift_Mime_HeaderSet is declared but never used. You should remove the use statement.
use Mandrill;
use \Swift_Events_EventDispatcher;
use \Swift_Events_EventListener;
use \Swift_Events_SendEvent;
use \Swift_Mime_HeaderSet;
use \Swift_Mime_Message;
use \Swift_Transport;
use \Swift_Attachment;
class MandrillTransport implements Swift_Transport {
in SwiftMailer/MandrillTransport.php, line 10
Posted from SensioLabsInsight