HubSpot Sales is able to track email opens by embedding a one pixel invisible image into the message once it is sent. When your recipient views the email and the images load, we send you a notification that the email has been opened. You can see this information in your HubSpot Sales stream as well.HubSpot Sales tracking can sometimes be deactivated if the recipient's email client doesn't allow the tracking pixel to load. For instance, if the recipient formats their email in plain text, if their email client doesn't automatically download images, or if their corporate filter strips away all images for incoming emails. This shouldn't happen often, but can happen from time to time.
If you have not already, learn how to install the HubSpot Sales email extension in your email client here.