How to set the cron?
1. The plugin scans for new emails to send every 10 minutes, but only if it has visitors. There is a 1 hour threshold, so all emails will be sent if you have at least 1 visitor per hour.
2. More reliable option is to set an automatic cron, which would call "yourwebsite.com/wp-cron.php" every 10 minutes (*/10 * * * *)