I've set up a simple Contact Us form using the built-in Form block. I checked the option to send an email notification on form submission but receive a nasty error when the form is submitted.

The error message is as follows:
Uncaught exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail' in /var/www/html/home/concrete/libraries/3rdparty/Zend/Mail/Transport/Sendmail.php:105 Stack trace: #0 /var/www/html/home/concrete/libraries/3rdparty/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail() #1 /var/www/html/home/concrete/libraries/3rdparty/Zend/Mail.php(973): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail)) #2 /var/www/html/home/concrete/helpers/mail.php(167): Zend_Mail->send() #3 /var/www/html/home/concrete/blocks/form/controller.php(329): MailHelper->sendMail() #4 [internal function]: FormBlockController->action_submit_form() #5 /var/www/html/home/concrete/models/block.php(262): call_user_func_array(Array, Array) #6 /var/www/html/home/concrete/startup/process.php(199): Block->passThruBlock('submit_form') #7 /var/www/html/home/concrete/dispatcher.php(204): require('/var/www/html/h...') #8 /var/www/html/home/index.php(2): require('/var/www/html/h...') #9 {main} thrown in /var/www/html/home/concrete/libraries/3rdparty/Zend/Mail/Transport/Sendmail.php on line 105

Any help is greatly appreciated.