I have recently upgraded to Magento 126.96.36.199. After upgrade, the invoice and ship emails are not working from the admin console. Clicking on 'Submit Invoice' (with 'Email copy of invoice' checked) takes to a blank 500 page.
After debugging, it seems that
$mailer->send() method is causing the issue. The method is not returning. The method is present in code/core/Mage/Sales/Model/Order/Invoice.php.
Any clue on what could be the issue ?