Bulk Email Newsletter Submission - no email limit SwiftMailer

Hi,

I have an action on my controller that triggers email from the database. I did some tests on my machine by inserting in the table only my own email, I got up to 1000. I only managed to receive 750 emails in my box. What can it be?

 $usuarios = Doctrine_Core::getTable('tbmailing')->createQuery('u')->select('u.email')->where('u.ativo = ?', 'Y')->execute();

    foreach ($usuarios as $usuario)
    {
        $sair = '<p style="font-family: Arial, Helvetica, sans-serif; font-size:x-small;">'
                . 'Se você deseja cancelar o recebimento deste boletim, '
                . '<a href="'.$rota.'/mailing/unsubscribe/'.base64_encode($usuario->getEmail()).'">'
                . 'acesse aqui.'
                . '</a>'
               . '</p>';

        $message = $this->getMailer()->compose(
            array("[email protected]" => "Meu Site"),
                $usuario->getEmail(),
                'Meu Site',
                '<img src="http://www.meusite.gov.br/images/informativoHeader.jpg"/>'.'<br><br>'.$body.'<imgsrc="http://www.meusite.gov.br/images/rodape.jpg" border="0"/>'
                .$sair
            );

        $message->setContentType("text/html");

        $this->getMailer()->send($message);
    }

I use SwiftMailer from Symfony framework, in version 1.4.