When designing emails, most companies don't give much thought to customer loyalty issues. Getting people to open and click dominate the process. However, there are a few simple procedures that prevent unsubscribes and signal our audience that we are willing to earn their attention and loyalty. 

• Use a calendar to establish an emailing plan that customers can learn to expect
• Segment customers by behavior, starting with the most loyal VIP types
• Study unsubscribers and develop a heartfelt re-engagement campaign–but do let people unsubscribe quickly.

SendGrid Blog: What is Retention Marketing? And How to Use it in Your Email Program, 2018-Jul-13 by Kelsey Bernius

You don’t have to dive head first and try out every last retention email campaign tactic. In fact, it’s better to start out and test one strategy at a time so you can measure your results before implementing more permanent programs.