Question:

How does Approved Email track the number of times an email is opened?

Answer:

Opens are tracked by including a transparent .png file in the email. It is a 1px by 1px image that, when rendered, registers an open event for that email which is stored in the Open_Count_vod__c field on the Sent_Email_vod__c object.

Items to Note:

• This tracking only works if there is an HTML component to the email (text-only emails do not track opens).
• Many email service providers disable images by default. If so, the tracking data is only returned if the recipient clicks on the Display Images button in the email.
• Opens are not tracked by individual users. It is simply a tabulation of the number of times the image is rendered. As such, there is no way to tell who opened the email. 

Related Documentation:

Mailgun Documentation: Tracking Opens