I run three Magento web stores. All the orders e-mails are working just fine, with the exception of 7 e-mails that didn't even got sent from the webserver. Upon investigating the issue I discovered the following logs:
2013-08-08T11:44:59+00:00 ERR (3): exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. ' in /var/www/website.com/lib/Zend/Mail/Transport/Sendmail.php:137
Aug 8 12:44:59 bv-webserver postfix/sendmail: fatal: email@example.com(33): No recipient addresses found in message header Aug 8 12:44:59 bv-webserver postfix/postdrop: fatal: uid=33: malformed input
This is very strange, because hundreds of e-mails were sent just fine and only these 7 got this exception. In some cases the clients just purchased again from the store and then received the e-mail as expected.
I thought it could be a resource problem. But I run the newrelic agent and the server is at 20% load at most during peak hours (both memory and processors).
Edit: Forgot to mention, I run nginx with php-fpm.