Magento Zend Mail using Google SMTP - magento

Question

I'm having troubles with mail sending in Magento: simply emails are not being sent in case of customer registration, order confirmation ecc.
I checked the exception log and this is what I found:
exception 'Zend_Mail_Protocol_Exception' with message '5.7.1 Username and Password not accepted. Learn more at
5.7.1 http://support.google.com/mail/bin/answer.py?answer=14257 q42sm2096509eem.14 - gsmtp
' in /srv/www/htdocs/lib/Zend/Mail/Protocol/Abstract.php:431
Stack trace:
#0 /srv/www/htdocs/lib/Zend/Mail/Protocol/Smtp/Auth/Login.php(95): Zend_Mail_Protocol_Abstract->_expect(235)
#1 /srv/www/htdocs/lib/Zend/Mail/Protocol/Smtp.php(217): Zend_Mail_Protocol_Smtp_Auth_Login->auth()
#2 /srv/www/htdocs/lib/Zend/Mail/Transport/Smtp.php(200): Zend_Mail_Protocol_Smtp->helo('localhost')
#3 /srv/www/htdocs/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Smtp->_sendMail()
#4 /srv/www/htdocs/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#5 /srv/www/htdocs/app/code/core/Mage/Core/Model/Email/Template.php(465): Zend_Mail->send(Object(Zend_Mail_Transport_Smtp))
#6 /srv/www/htdocs/app/code/core/Mage/Core/Model/Email/Template.php(517): Mage_Core_Model_Email_Template->send(Array, Array, Array)
#7 /srv/www/htdocs/app/code/core/Mage/Core/Model/Email/Template/Mailer.php(79): Mage_Core_Model_Email_Template->sendTransactional('customer_create...', 'general', Array, Array, Array, '1')
#8 /srv/www/htdocs/app/code/core/Mage/Customer/Model/Customer.php(650): Mage_Core_Model_Email_Template_Mailer->send()
#9 /srv/www/htdocs/app/code/core/Mage/Customer/Model/Customer.php(577): Mage_Customer_Model_Customer->_sendEmailTemplate('customer/create...', 'customer/create...', Array, '1')
#10 /srv/www/htdocs/app/code/core/Mage/Customer/controllers/AccountController.php(344): Mage_Customer_Model_Customer->sendNewAccountEmail('confirmation', 'http://test.col...', '1')
#11 /srv/www/htdocs/app/code/core/Mage/Core/Controller/Varien/Action.php(419): Mage_Customer_AccountController->createPostAction()
#12 /srv/www/htdocs/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('createpost')
#13 /srv/www/htdocs/app/code/core/Mage/Core/Controller/Varien/Front.php(176): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#14 /srv/www/htdocs/app/code/core/Mage/Core/Model/App.php(354): Mage_Cor_Controller_Varien_Front->dispatch()
#15 /srv/www/htdocs/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#16 /srv/www/htdocs/index.php(87): Mage::run('', 'store')
#17 {main}
I didn't know about the Zend Mail Protocol, anyway, from what I see, this is trying to use SMTP authentication on a Google server; the fact is I have not installed any extension to use SMTP in Magento, so I have never set up any username or password; besides I'm using localhost in Magento mail sending settings, so anyone has an idea of what happened and can suggest me a workaround?
Thank you!