The reading pane can be part of the equation, but there’s more.
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You probably already realize that it’s possible to embed images in HTML-formatted email. Images can be carried within the email itself, and that’s relatively safe. But images can also be embedded in email as references, meaning that the image is not actually part of the email, but rather it’s loaded from somewhere on the internet when you view the mail. The mere act of loading that image can be used by whomever sent you the email containing it to verify that you did in fact open the mail.
The Reading or Preview Pane is a particularly interesting because if your inbox is empty, it could automatically display the contents of the first message that arrives. Even if you’re not there.
Regardless of how you display it, if the images are shown, then there’s a possibility that the spam in your inbox can be telling the senders that the email address they just tried is real, ripe, and ready for more spam.
That’s why in recent versions Outlook and Outlook Express default to not displaying images unless you explicitly say it’s ok – either for a specific message or for senders you explicitly add to your “safe senders list”.