So when your email client attempts to "draw" the image it requests that
jpg file which makes a request to the server and that's how they know
you opened it.

I think this is the more common technique.  Originally done with 1 x 1 pixel
"images", I've sometimes wondered how often this is done with "legitimate"
sized images which would be harder for a web washer style process to strip


PS - It is also one of the reasons I intentionally continue to use a text
only email client which does not render HTML... (except for these lists I
now use gmail)

This thread ...


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