We track Email Open by putting a tiny invisible image at the bottom of your HTML message. When Email is opened this imaged gets downloaded and we track this download as Email Open event.
Email Tracking will work only in HTML messages because we obviously can’t embed an image in plain text message. Also a lot of Email clients prevent downloading of images in HTML. You will see some text placeholder in place of images in such messages. This is a privacy measure that is used very widely by email applications. If this privacy measure is in place at your email recipient’s email app, then we will not be able to track Email Open unless the recipient explicitly download the images. You obviously can’t ask your recipients to change their privacy settings.
So the Email Open measurement may not be accurate. You should instead measure Click action on links within Email. Click is a better measurement from reliability as well as lead engagement perspective.
In HTML message you can hyperlink any text. Any click to the link will be tracked. In plain text message you should provide full URL in plaintext including the “http://”. For example if you want to hyperlink to www.yourwebsite.com then you should write the URL as http://www.yourwebsite.com .