When looking in the logs, I see
exception 'Exception' with message 'This letter cannot be sent.' in /var/www/app/code/core/Mage/Core/Model/Email/Template.php:398, which leads to:
public function isValidForSend()
{
return !Mage::getStoreConfigFlag('system/smtp/disable')
&& $this->getSenderName()
&& $this->getSenderEmail()
&& $this->getTemplateSubject();
}
Disable Email Communications is already set correctly. What else could it be? The user enters all of these details. We've been searching for hours.
Thanks in advance!
Edit: It turned out that the template subject was always returning null. After explicitly assigning the email templates in the back-end instead of using the defaults from the locale folder, it started working again. I still haven't been able to figure out just where it comes from, but it works.
Best Answer
Change the code for a moment to this:
Enable logging and after replicating the scenario check in var/log/email_issue.log.