我正在尝试在我的magento项目中发送邮件。我收到以下错误：
2016-12-22T07:05:19+00:00 ERR (3):
exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in /home2/ultrasou/public_html/daniel/lib/Zend/Mail/Transport/Sendmail.php:137
Stack trace:
#0 /home2/ultrasou/public_html/daniel/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 /home2/ultrasou/public_html/daniel/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Model/Email/Template.php(451): Zend_Mail->send()
#3 /home2/ultrasou/public_html/daniel/app/code/local/Aschroder/SMTPPro/Model/Email/Template.php(32): Mage_Core_Model_Email_Template->send('hemanthsemtech@...', NULL, Array)
#4 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Model/Email/Template.php(508): Aschroder_SMTPPro_Model_Email_Template->send('hemanthsemtech@...', NULL, Array)
#5 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Contacts/controllers/IndexController.php(104): Mage_Core_Model_Email_Template->sendTransactional('contacts_email_...', 'custom2', 'hemanthsemtech@...', NULL, Array)
#6 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Contacts_IndexController->postAction()
#7 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('post')
#8 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#9 /home2/ultrasou/public_html/daniel/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#10 /home2/ultrasou/public_html/daniel/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#11 /home2/ultrasou/public_html/daniel/index.php(87): Mage::run('', 'store')
#12 {main}
如何解决这个问题？