Emails are considered opened when they meet one of the following criteria:
  1. The recipient enables or downloads the images in the email
  2. The recipient interacts with the email by clicking on a link

If the email client has a preview pane enabled, then when the email is displayed in the preview, the email is only considered open if the images are downloaded. 

In regards to the downloaded images: open rates are typically tracked using a transparent 1x1 pixel, or small transparent tracking image embedded in outgoing emails. When the client or browser used to display the email requests that image, then an "open" is recorded for that email by the image's host server.

For more information about email marketing, please visit our Non-Profit Industry Expert Blogs at www.npengage.com
Including, this blog titled Don't be Fooled by Message Open and Click-Through Rates

Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites, and we cannot assist with implementing resolutions from these websites.