mail not working

hello,

Amember not sending any email to customer. in logs error is showing is "Unable to send mail.".
and the completed error is

Exception Zend_Mail_Transport_Exception
Zend_Mail_Transport_Sendmail->_sendMail [ library/Zend/Mail/Transport/Abstract.php : 348 ]
Zend_Mail_Transport_Abstract->send [ library/Am/Mail.php : 387 ]
Am_Mail_Queue->send [ library/Zend/Mail.php : 1194 ]
Zend_Mail->send [ library/Am/Mail.php : 118 ]
Am_Mail->send [ library/Am/Mail/Template.php : 176 ]
Am_Mail_Template->send [ application/default/models/User.php : 389 ]
User->sendSignupEmail [ application/default/models/User.php : 374 ]
User->sendSignupEmailIfNecessary [ application/default/models/User.php : 353 ]
User->checkSubscriptions [ application/default/models/Invoice.php : 1243 ]
Invoice->addAccessPeriod [ application/default/models/Invoice.php : 1304 ]
Invoice->addPayment [ application/default/controllers/AdminUserPaymentsController.php : 911 ]
AdminUserPaymentsController->addpaymentAction [ library/Am/Controller.php : 139 ]
Am_Controller->_runAction [ library/Am/Controller.php : 116 ]
Am_Controller->dispatch [ library/Zend/Controller/Dispatcher/Standard.php : 308 ]
Zend_Controller_Dispatcher_Standard->dispatch [ library/Zend/Controller/Front.php : 954 ]
Zend_Controller_Front->dispatch [ library/Am/App.php : 2019 ]
Am_App->run [ index.php : 92 ]