Hi!

There are two main ways of achieving this. The first is present in all Webnodes installation. This solution is to create a class that implements the INewsletterActionMethods interface. It contains methods that get called when a recipient reads, clicks a link or unsubscribes. Each newsletter has an ActionClassName property where you specify the classname of the class you've created. In your class, you can store which recipients have read the various newsletters. 

The second option is to buy the new Segmentation module. It stores all the actions mentioned above (if you enable them in the settings of your Webnodes installation). The current version of Webnodes and the segmentation module doesn't provide a report that lists users that have or have not read a newsletter, but the data is stored in the database, so you could create your own report from the data. The next version of the Segmentation module, due out at the end of the year, will include more functionality around this topic. 

Hope this helps! Let me know if anything is unclear!