When your email is sent, a piece of code automatically gets included that requests an invisible image from our web server. When a recipient opens the email, the image gets downloaded, and that download gets recorded as an “open” for that specific address.
Open rates are not 100% accurate, however. Under the best of circumstances, opens get recorded when the recipient’s email client is able to display HTML images (it is not possible to record opens for text-only messages), AND the recipient’s “view images” settings are turned on (some people prefer to read emails without viewing images).
Recording an open can get muddied when the recipient’s email client automatically displays your email in a preview pane, which downloads the image in the process, but gives no indication that the recipient actually read the email or clicked on anything.
That’s why open rates should only be used as a general guide to help measure trends, and not for hard and fast numbers.