protected function authPlain()
    {
        $digest = base64_encode( "\0{$this->user}\0{$this->password}" );
        $this->sendData( "AUTH PLAIN {$digest}" );
        if ( $this->getReplyCode( $error ) !== '235' )
        {
            throw new ezcMailTransportSmtpException( 'SMTP server did not accept the provided username and password.' );
        }

        return true;
    }