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I'm guessing if you used a popular web-based email service, or any browser email client, then this would be possible.

Possibly, but you'd still have to (try to) "render" it in your browser by opening the mail.

On a similar note, your web mail could fetch images in emails ahead of time, but that would still be out of your browser's context

note that the url is inside an <IMG> tag.

Remember...... 2008. Many people still had "auto download pictures" enabled in their email.