Magento CE 1.9.1.1 not sending sales emails

Question

We are having issues with Magento not sending emails out. The emails having issues are the sales email which are logged into core_email_queue . None of these are sending. All other emails send fine through my server.

I am aware that CE1.9.x send emails through cron and my cron.php is running fine. AOE scheduler shows the cron is running. Only the emails are not being sent. In exception.log I see this issue which is exactly when the emails stopped going out and it repeats.

How can I fix this? It's a fresh install of CE1.9.1.1 and I reinstalled just to be sure.

2015-04-30T01:35:03+00:00 ERR (3): 
exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in /home/nli/catalog/lib/Zend/Mail/Transport/Sendmail.php:137
Stack trace:
#0 /home/nli/catalog/lib/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 /home/nli/catalog/lib/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 /home/nli/catalog/app/code/core/Mage/Core/Model/Email/Queue.php(241): Zend_Mail->send()
#3 [internal function]: Mage_Core_Model_Email_Queue->send(Object(Aoe_Scheduler_Model_Schedule))
#4 /home/nli/catalog/app/code/community/Aoe/Scheduler/Model/Observer.php(79): call_user_func_array(Array, Array)
#5 /home/nli/catalog/app/code/core/Mage/Core/Model/App.php(1338): Aoe_Scheduler_Model_Observer->dispatch(Object(Varien_Event_Observer))
#6 /home/nli/catalog/app/code/core/Mage/Core/Model/App.php(1317): Mage_Core_Model_App->_callObserverMethod(Object(Aoe_Scheduler_Model_Observer), 'dispatch', Object(Varien_Event_Observer))
#7 /home/nli/catalog/app/Mage.php(448): Mage_Core_Model_App->dispatchEvent('default', Array)
#8 /home/nli/catalog/cron.php(76): Mage::dispatchEvent('default')
#9 {main}

Answer 1

I think following post can help you.

Magento:exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail

There is a chance that your from address can have extra character that seems invalid.

Answer 2

you can check this link out.

http://www.appseconnect.com/magento/magento-1-9-bug-sales-order-emails-does-not-hit-instantly-it-sends-when-magento-cron-jobs-is-initiated/

This will help you to get rid of late sending emails.