If you have any issues with your delayed email notifications not going out at the exact time they’re scheduled to, then it could be related to your website’s traffic.
Our plugin uses WP-Cron to delay emails. It’s usually quite reliable, but there are times where it can hiccup depending upon either too little traffic or too much traffic. One thing that can help in improving reliability of the WP-Cron would be to set up a recurring 5-minute Cron job based upon your webhost server instead of having the CRON driven by site traffic (the default way).
To resolve this issue, we would recommend working with your webhost to setup a recurring Cron job with them. Doing so is unique to each webhost, but here’s a simple guide that can provide the overall instructions for getting this setup:
https://www.siteground.com/tutorials/wordpress/real-cron-job/