There can be several reasons for a high number of opens. The registration of "opens" is done by use of a tracking pixel, which is sent with every e-mail. When an email is opened, this tracking pixel is downloaded. For this to the happen, the email client of the recipient must allow the downloading of images.

However, not every mail client works the same. For example, in some mail clients, the tracking pixel is already downloaded when the preview is displayed, without the specific mail being opened. As a result, it may seem that the e-mail has been openened often, while really only the preview has been loaded. It is also possible that the recipient has forwarded your email. Each time this forwarded email is opened, "opens" are registered on the original recipient of your mailing.

Because of these reasons, there are technical limitations to the accurate measurement of "opens" by one and the same person, although in most cases the open rate does give a good indication.