If you use Magento, then you depend on its email functionality to send order confirmations, invoices, and shipping confirmations to your clients. Aside from technical people, many Magento users do not know how Magento sends email. While knowing how Magento sends email may not be essential to running your online store, it becomes extremely useful in troubleshooting email issues related to Magento.
Magento is built on top of the Zend Framework using the Zend_Mail class to send mail. The main class that does this work is Zend_Mail_Transport_Sendmail.php. Zend_Mail_Transport_Sendmail.php is built on top of PHP Mail. PHP Mail in turn uses the server’s sendmail script to attempt to send mail.
It should be noted that it is not possible to use Zend_Mail_Transport_Sendmail.php to specify SMTP settings.
Basically, a vanilla installation of Magento will be using sendmail to send email.
* Introduction to Zend Mail http://www.zendcasts.com/introduction-to-zend_mail/2010/02/
* PHP Mail http://php.net/manual/en/function.mail.php