Open tracking is enabled per SMTP user, authenticated IP address, or API key. To enable open tracking for an SMTP user, visit the "Settings > SMTP Users" section of your account’s dashboard:
Once enabled, you'll then have the ability to see if an email was opened, as well as an estimate of how long it was opened, from the "Reports > Activity" section. This information can be viewed from the "Timeline" tab upon selecting the recipient address via the right-hand column. If an email is opened multiple times by the recipient then multiple ‘Opened’ events will be added to the timeline.