Emails not sending in Magento 1.3.1

 As a temporary measure a fix can be issued in the form of editing core code.

1) Enter the root directory of your Magento store and enter

# mkdir app/code/local/Mage
# mkdir app/code/local/Mage/Core
# mkdir app/code/local/Mage/Core/Model
# mkdir app/code/local/Mage/Core/Model/Email
# cp app/code/core/Mage/Core/Model/Email/Template.php  app/code/local/Mage/Core/Model/Email/

2) Using your favourite editor open up Template.php and make changes at these lines

ini_set('SMTP', Mage::getStoreConfig('system/smtp/host'));
        ini_set('smtp_port', Mage::getStoreConfig('system/smtp/port'));
        $mail = $this->getMail();
        if (is_array($email)) {
            foreach ($email as $emailOne) {
+               # Fix for mini_sendmail
+                # $mail->addTo($emailOne, $name);
+                $mail->addTo($emailOne);
-                $mail->addTo($emailOne, $name);
            }
        } else {
+           # Fix for mini_sendmail
+           # $mail->addTo($email, '=?utf-8?B?'.base64_encode($name).'?=');
+           $mail->addTo($email);
-           $mail->addTo($email, '=?utf-8?B?'.base64_encode($name).'?=');
        }
        $this->setUseAbsoluteLinks(true);
        $text = $this->getProcessedTemplate($variables, true);

$mail->setSubject('=?utf-8?B?'.base64_encode($this->getProcessedTemplateSubject($variables)).'?=');
+       # Fix for mini_sendmail
+       # $mail->setFrom($this->getSenderEmail(), $this->getSenderName());
+       mail->setFrom($this->getSenderEmail());
-       $mail->setFrom($this->getSenderEmail(), $this->getSenderName());

3) Recompile (if using extension) to update includes
It is not a perfect solution, but should provide a fix until we find an alternative to mini_sendmail.

For more information click here

If the issue is not in 1.3.1 version then the solution can be found here