Internet Advertising Report: Microsoft Outlook 2003 Puts Open Rates in Question:

“The issue lies in the way Outlook 2003 handles images and links in HTML messages. In previous versions, graphics and links were delivered to the computer and appeared in Outlook’s preview pane (in default mode). Now, HTML, images and rich media are cached on the server and downloaded only when the message is actually opened. This protects users from viewing possibly objectionable images; prevents invisible executables from opening in preview mode; and scotches directory harvesting of e-mail addresses. Users must either manually click to load a message’s images and links, or turn off the image-caching feature in the preferences. “