Email open and click tracking is built-in, using Ahoy::Email to gather the data.
By default these features are disabled for privacy reasons. You can enable them on the Site Settings page in the admin area.
Even when click-tracking is enabled in the settings, clicks will not be tracked for emails whose links might contain 'sensitive' data - e.g. password reset emails - but you can track opens for these emails.
This feature is likely to be moved into a plugin at some point.