If you are using HubSpot Sales with Outlook on a Windows computer, you can install the HubSpot Sales for Windows tray app to receive real time notifications when your emails are opened.
Once you've installed the app, you can enable it from the Outlook extension's configuration menu. It will look like this on your desktop:
Right-click on the icon in your tray to pause notifications, restart the app, or access other options.
For instructions on downloading HubSpot Sales, check out this article.