Just dig.
Your Exception is thrown here: app/code/core/Mage/Contacts/controllers/IndexController.php:107
therefore $mailTemplate->getSentSuccess() must be false
It is set false here: app/code/core/Mage/Core/Model/Email/Template.php:479
but the mail is going out, so this code is executed and send() returns false:
app/code/core/Mage/Core/Model/Email/Template.php:506
$this->setSentSuccess($this->send($email, $name, $vars));
this means, this code is executed:
app/code/core/Mage/Core/Model/Email/Template.php:453
try {
$mail->send();
$this->_mail = null;
}
catch (Exception $e) {
$this->_mail = null;
Mage::logException($e);
return false;
}
Most likely Zend_Mail_Transport_Sendmail is used, so it continues here:
\Zend_Mail_Transport_Sendmail::_sendMail
What ever happens there, I would think throw new Zend_Mail_Transport_Exception('Unable to send mail. ' . $this->_errstr); is thrown.
This means for me, you should add some logging here \Zend_Mail_Transport_Sendmail::_handleMailErrors ,because of the catched and newly thrown Exception (app/code/core/Mage/Core/Model/Email/Template.php:453) all the errors descriptions are gone.
But the error should be logged in exception.log. Did you check it? If not, read this: Fundamentals of debugging Magento
Best Answer
After searching lot's of thing i found solution for this problem. This issue can solve by disabling mailQueue.
Change Line
To
for more details visit here
But there may be case that it can affect other mail module