Robin Sloan on transparent tracking pixels, the invisible images that can show the sender precisely when an email was opened.

In the marketing context, I think this kind of data collection is okay — barely — but only in the aggregate, i.e. to judge the overall performance of a newsletter.
In the personal context, it’s shockingly presumptuous. An email isn’t a letter, but even so: imagine unsealing a letter, and a winking electronic transponder slips out. You would have questions for your correspondent!

and here is my favorite part:

Anyway, I wish Gmail offered an option less passive than simply declining to display the tracking pixel — maybe some way to send a fart sound streaming back into the sender’s inbox…

LINK