While I haven't actually used it, an email analytics web app called Litmus claims to be able to track:
I'm curious about where it gets this information from. Most email clients i've seen don't even load external images without explicitly loading them, let alone javascript.
Even if a lot just support images, that wouldn't give away items 1-3.
Here's my best guess. As this link rockinthesixstring posted says, it relies on images but not javascript.
How long someone takes to read an email.
Place several images in the email that take a while to load, if the email is read for a long time, more are loaded.
Whether it is forwarded.
Is the image loaded from more than one IP/user agent?
Whether it is deleted.
The screenshots show this combined with glanced.
Whether it is printed.
Add a background image bug to a print stylesheet.
What email client was used to read it.
Check the user agent.