The open rate is tracked with the help of a hidden one-pixel image placed in the body of the email message. When an email recipient opens the email and when the one pixel image is downloaded, an "open" is recorded in your Campaigns database. When the recipient opens the email multiple times, the open count increases.

 

This tracking method is only used for HTML emails. For plain text emails, we can't use the same technique as we can't embed images in the email. In these situations, you can track the open rate using URLs. If you have URLs in your email campaign and if you have enabled the URL tracking feature, the open/click rate will be tracked when recipients click the URL.