Found this error in my error logs:
PHP Question2Answer email send error: You must provide at least one recipient email address.
It derives from the class.phpmailer.php:
if ((count($this->to) + count($this->cc) + count($this->bcc)) < 1) {
throw new phpmailerException($this->lang('provide_address'), self::STOP_CRITICAL);
}
It happened when I did a mass-mailing from the admin panel.
But checking my users, there are none that have empty email addresses. What could be the reason?