Instead of guessing what people want, just look at the data you already have. Many marketers have put retargeting to good use with ad networks but your presence in the inbox is much more powerful. There are two ways to do this.
First, you can track what your email subscribers click and show them ads based on that data. Or, you can track logged in users behavior on your site and trigger emails based on their activity. Airbnb has mastered this. When a logged in user views a listing but doesn’t actually rent, Airbnb will trigger an email to that user with more information on the listing as well as nearby rentals in the same price range. https://conversionxl.com/blog/email-retargeting-not-just-for-websites-display-networks-anymore/