Definitely check out Concierge. Product recommendations by emails.
App page >> https://apps.shopify.com/concierge
Product page >> https://concierge.sendicate.net/
Concierge is brand new but already has 100 installs and sold 70 products so far. All the reviews have been amazing and people really love the app. Just turn it on and watch the sales start coming in. Its also free!
I'm the developer so please let me know if you have any feedback.