Symfony 2通过谷歌的传输速度非常慢

In our production server we are facing with unexpected problem.

From yesterday requests with imply sending emails take SOMETIMES very long time. Some of that requests finish successfully at the end of second minute, some of them terminates with exception:

[2015-07-10 15:40:18] request.CRITICAL: Uncaught PHP Exception Swift_TransportException: "Expected response code 250 but got code "", with message """ at /var/www/html/project/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php line 386 {"exception":"[object] (Swift_TransportException: Expected response code 250 but got code \"\", with message \"\" at /var/www/html/project/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:386)"} []

Configuration is just ordinar:

mailer_transport: gmail
mailer_username: ***
mailer_user: ***
mailer_password: ***
mailer_from: ***

Sending also:

$message = \Swift_Message::newInstance()
    ->setSubject('Confirmation')
    ->setFrom($this->container->getParameter('mailer_from'))
    ->setTo($email)
    ->setBody($this->renderView('Bundle:emails:emailConfirmation.html.twig',  array('invoice' => $invoice)), 'text/html');
$mailer->send($message);

Pay attention that sometimes it works fine sometimes takes 2 mins sometimes brakes.

Other symptom is when it starting emailing for ever it's not possible to request the site with the same browser (it waits). Whereas other browser loads site as usual.

It's not depend on email volume. Even tiny emails of password reinit can cause that.

Any ideas ?

Thanks.