Once I got an external crons triggering wp-cron.php, this thing rocked. I gave it only 4 stars because I'm not sure if it handles bounced emails.
If you use a scheduled crons, make sure you add this line of code to your wp-config.php file:
You can then use your hosting crons job to trigger wp-cron.php like such:
wget -q -O - http://yourwebsite.com/wp-cron.php?doing_wp_cron >/dev/null 2>&1
Run it every five minutes and you should be good to go.