How do product recommendations work?
There are a three different types of recommendation triggers.
Related Products - Products will be recommended based off of Related Product data that is stored in Magento.
Cross-Sell - Products will be recommended based off of the Cross-Sell data that is stored in Magento.
Purchase Relation - Products will be recommended based off of order history. This doesn't rely on any data being populated, and MageMail simply calculates "people who bought X also bought Y" and makes recommendations based off of that data.
In all cases, customers who already purchased a given product will not receive a recommendation for it.
Recommended product content will only work in trigger emails with "recommendation" in the trigger type. For example: Recommendations will not work in Thank You or Abandoned Cart emails, but will work in Recommendation: Cross-sell emails.