i have this problem how to solve this

Mail Exception Occurred. Unable to send mail: Unable to send mail.
#0 /home/qmbmalis/public_html/concrete/libraries/3rdparty/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 /home/qmbmalis/public_html/concrete/libraries/3rdparty/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 /home/qmbmalis/public_html/concrete/helpers/mail.php(349): Zend_Mail->send(NULL)
#3 /home/qmbmalis/public_html/concrete/blocks/form/controller.php(405): MailHelper->sendMail()
#4 /home/qmbmalis/public_html/concrete/models/block.php(325): FormBlockController->action_submit_form()
#5 /home/qmbmalis/public_html/concrete/startup/process.php(325): Block->passThruBlock('submit_form')
#6 /home/qmbmalis/public_html/concrete/dispatcher.php(269): require('/home/qmbmalis/...')
#7 /home/qmbmalis/public_html/index.php(2): require('/home/qmbmalis/...')
#8 {main}
Template Used: block_form_submission
To: shpejtimhoxha@gmail.com
From: info@qmb-malisheva.org
Subject: Kontakti Form Submission
Body:
There has been a submission of the form Kontakti on through your concrete5 website.

e-mail
shpejtim@hoxha.com

mesazhi
tung shpejtim



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