How do I set up a trigger email notifying customers of a delay in shipping for everyone who has pre-ordered an item?

For tasks that required email notifications to be sent to our merchant's customers. We highly recommend Shopify Email Apps, whose primary focus is to assist Shopify merchants in triggering automated email effectively. Below are some of these Emails Solution listed on the Shopify App Store: 

1. Klaviyo

2. Seguno 

3. Back in Stock: Customer Alerts, SureSwift Capital

4. Back in stock - Restock notify