Web beacon(en.wikipedia.org)
en.wikipedia.org
Web beacon
https://en.wikipedia.org/wiki/Web_beacon
2 comments
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.
At least the Mozilla foundation is trying to to create an Opt-Out standard. But who says any of them will adopt this standard.