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The images are precached by Google so that this doesn't happen. The requested image doesn't come from its originating source.

For example, here is Basecamp's logo as it's displayed by Gmail:

https://ci5.googleusercontent.com/proxy/YBj6RKkd7c8dtR81gfn-...

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Google acts as a sort of proxy for those images, so by setting certain http headers, the marketers can make the proxy access the original image every time the email is being opened. So they can track the time you open their email.

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