Magento 1.5.1 Emails not working

Hello there, Recently I have created magento store site and applied custom theme on development server and everything including email was working as expected. when I need to make this site live I've copied codebase and replicate database and change config.xml for database setting. Everything was working fine but I found yesterday that My live site not seding any emails not for sale or not for even reset password.

Any comments or suggestions ?

exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in /home/rmansuri/webapps/wmm/lib/Zend/Mail/Transport/Sendmail.php:137

Stack trace:

#0 /home/rmansuri/webapps/wmm/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()

#1 /home/rmansuri/webapps/wmm/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))

#2 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Model/Email/Template.php(384): Zend_Mail->send()

#3 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Model/Email/Template.php(437): Mage_Core_Model_Email_Template->send('riz@webbyfox.co...', 'riz mansuri', Array)

#4 /home/rmansuri/webapps/wmm/app/code/core/Mage/Customer/Model/Customer.php(580): Mage_Core_Model_Email_Template->sendTransactional('customer_passwo...', 'support', 'riz@webbyfox.co...', 'riz mansuri', Array)

#5 /home/rmansuri/webapps/wmm/app/code/core/Mage/Customer/controllers/AccountController.php(522): Mage_Customer_Model_Customer->sendPasswordReminderEmail()

#6 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Controller/Varien/Action.php(418): Mage_Customer_AccountController->forgotPasswordPostAction()

#7 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(253): Mage_Core_Controller_Varien_Action->dispatch('forgotpasswordp...')

#8 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Controller/Varien/Front.php(176): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))

#9 /home/rmansuri/webapps/wmm/app/code/core/Mage/Core/Model/App.php(340): Mage_Core_Controller_Varien_Front->dispatch()

#10 /home/rmansuri/webapps/wmm/app/Mage.php(627): Mage_Core_Model_App->run(Array)

#11 /home/rmansuri/webapps/wmm/index.php(80): Mage::run('', 'store')

#12 {main}