[seahorse]

Maybe Gmail does, but not all mail services do.

It wouldn't be that hard to add a 1x1 invisible pixel in the message footer and see how well it works.

[MichaelGG]

Gmail no longer does. They download the images for you, so your IP isn't leaked. I'm not sure if this is done beforehand (removing all info from the load), or after you open the email.

[Mizza]

Oh, interesting. So open tracking still works, just doesn't come directly from the client.

Interesting.. maybe this is do-able after all.. /mail/{{mail_id}}/{{open_tracker_id}}.gif

[jrs235]

Gmail pre-downloads images and caches them. This causes false positives that enails have been read when in fact they may not have been.