How do activities work with smart emails?

You can create a Salesforce task or message when you set up a smart email mapping. You can only do this if your root object allows activities.  Creating tasks or messages is necessary if you want to see tracking activity in our lightning component. Without a task or message, you will not be able to see transactional tracking information.

It is also worth noting that an activity will be created for each recipient.  So if you email a person and include a cc two activity records will be created.