public function setContentTransferEncoding($encoding)
    {
        switch ($encoding) {
            case '7bit':
                $encoder = \Swift_Encoding::get7BitEncoding();
                break;

            case '8bit':
                $encoder = \Swift_Encoding::get8BitEncoding();
                break;

            case 'base64':
                $encoder = \Swift_Encoding::getBase64Encoding();
                break;

            case 'qp':
                $encoder = \Swift_Encoding::getQpEncoding();
                break;

            default:
                throw new SwiftMailerException('Invalid encoding name provided.
												Valid encodings are [7bit, 8bit, base64, qp].'
                );
                break;
        }

        $this->message->setEncoder($encoder);

        return $this;
    }