What is the best model for a recommendation system using implicit ratings?
There are different approaches for your case, but I believe you can employ approaches that are based on the self-attention mechanism. There numerous studies that show they perform better than the latent outcomes that can be achieved with SVD. For instance, you can see the paper of SAIN which its code is also open source. In the paper, you can see that they have shown their approach has better performance than SVD and usual self-attention mechanisim. By the way, it is based on self-attention.
If I want to summarize the method, it uses the contents of the products and the items and finds the interactions between the words, and it outputs the score that a user may assign to a product. It has good performance.
Due to the fact that you have the contents of mails, this approach may help you.