Outbound HTML emails are tracked by default if your organization uses HTML email templates. You can select or deselect this setting at Setup | Customize | Activities | Activity Settings to control email tracking. If you disable email tracking, email tracking information for your organization is no longer stored. However, the HTML Email Status related list remains on page layouts and email tracking reports remain on the Reports tab.
After sending an HTML email, you can track the date it was first opened, the number of times it was opened, and the date it was most recently opened.