Script error in WordPress

Hello,

I have a site on WordPress, there is an option that client can send to me a short form (name, message and email). When he fills needed fields he gets this message:

Fatal error: Uncaught exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail' in /cms/library/Zend/Mail/Transport/Sendmail.php:105 Stack trace: #0 /cms/library/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail() #1 /cms/library/Zend/Mail.php(720): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail)) #2 /php/modules/index/classIndex.php(533): Zend_Mail->send() #3 /php/modules/index/classIndex.php(620): Index->send_question2() #4 /php/modules/index/classIndex.php(56): Index->getContent() #5 /index.php(3): Index->__construct() #6 {main} thrown in /cms/library/Zend/Mail/Transport/Sendmail.php on line 105

Could anyone help or give a small advice, I dont know what can be wrong…

Thank you
justcef