There are a couple of other "automated e-mail marketing" products available in the marketplace and some free plugins available from the Zen Cart website that will send your customers an email some number of days after a specific event occurs. The commercial offerings that I've reviewed are, in my opinion, overly complex while the free plugins are very narrow in their support (e.g. only sending an automated email for a survey a number of days after purchase). The Automated Email Engine (AEE) fills that gap by providing a customizable system for automating after-purchase emails to your customers.
With the AEE, you begin by configuring the order-related events that are important to your store. When you configure an event, you define the condition under which the AEE starts its countdown to performing an action. Once you've configured the event conditions, you can manage the orders that are currently waiting or ready to be processed.
Full details on the plugin's operation are documented in its readme, available via click on the Readme icon below and to the right.