
Web beacon - imdsm
https://en.wikipedia.org/wiki/Web_beacon
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randomerr
So the beacon tracks a user by their IP address and browser client
information. Advertisers/marketers have you put a 1px x 1px transparent GIF or
PNG image on your pages or in an email blast. The image is hosted on their
remote server. Then the remote server put your information into a database. By
cross-referencing your IP browser agent information they can track you by
without creating a cookie, local storage, or indexed database anymore.

At least the Mozilla foundation is trying to to create an Opt-Out standard.
But who says any of them will adopt this standard.

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p0peax
This is a reason to be wary of loading external images by default in mail. I
don't know if they do, but spammers could quickly identify active e-mail
addresses using this technique, so simply opening a mail could mean you would
get even more spam, because your e-mail address is now marked as active.

