XF 1.5 How to resolve email error [Zend_Mail_Transport_Exception]

[JacquiiDesigns]

Using XenForo 1.5.16 and have been receiving the following server error as quoted from the server log (website url redacted...):

Server Error Log

Error Info
Zend_Mail_Transport_Exception: Email to ******@gmail.com failed: Unable to send mail. - library/Zend/Mail/Transport/Sendmail.php:137
Generated By: Unknown Account, 14 minutes ago

Stack Trace
#0 /home/******/******.org/library/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail()
#1 /home/******/******.org/library/Zend/Mail.php(1194): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail))
#2 /home/******/******.org/library/XenForo/Model/MailQueue.php(79): Zend_Mail->send(Object(Zend_Mail_Transport_Sendmail))
#3 /home/******/******.org/library/XenForo/Deferred/MailQueue.php(10): XenForo_Model_MailQueue->runMailQueue(7.9999961853)
#4 /home/******/******.org/library/XenForo/Model/Deferred.php(295): XenForo_Deferred_MailQueue->execute(Array, Array, 7.9999961853, '')
#5 /home/******/******.org/library/XenForo/Model/Deferred.php(429): XenForo_Model_Deferred->runDeferred(Array, 7.9999961853, '', false)
#6 /home/******/******.org/library/XenForo/Model/Deferred.php(374): XenForo_Model_Deferred->_runInternal(Array, NULL, '', false)
#7 /home/******/******.org/deferred.php(23): XenForo_Model_Deferred->run(false)
#8 {main}

Request State
array(3) {
["url"] => string(38) "http://******.org/deferred.php"
["_GET"] => array(0) {
}
["_POST"] => array(4) {
["_xfRequestUri"] => string(48) "/conversations/lets-see-if-its-working-now.3880/"
["_xfNoRedirect"] => string(1) "1"
["_xfToken"] => string(8) "********"
["_xfResponseType"] => string(4) "json"
}
}

Absolutely No emails are being sent from the XenForo script. The error produces an entry in the server log whether the email that should be sent is a email verification or new post notification. It's an obvious server error of some sort ... Or so that I think. But I wonder - does anyone know how to resolve this error?

I'll definitely appreciate all assistance in resolving this issue.
Thanks!

J.

 

[Paul B]

https://xenforo.com/community/resou...ting-guide.5086/updates#resource-update-18729