So, ‘Litmus’, a web app for testing emails and webpages across browsers and email clients, has a proprietary method that they claim is able to track not just opens, clicks, browsers, etc (standard with an embedded image and pass-through link tracking.)
What’s unique is they claim that they are able to track what actions the end user took, how long the end user read it for, and if they deleted or forwarded the email. They claim they do this without JavaScript, and purely using embedded images. They claim that the method works across most major email clients.
What could they be doing to track this? Obviously, if they’re doing it with third party applications that they don’t control, whatever they are doing should be replicable.
I’m thinking that they realized that when an email client forwards or deletes an email, it ‘opens’ the email in a different way then normal, creating a unique user string on the server log of some kind? I’m grasping at strings, though.
http://litmusapp.com/email-analytics
Details here http://litmusapp.com/help/analytics/how-it-works
EDIT: It also looks like they track Prints. Maybe they do this by tracking calls to the ‘print’ css?
It’s all done with good ol’ image bugs. Breaking down how they find out…
Which client was used: Check the user-agent
Whether an email was forwarded: Done by attaching image bugs to divs that are loaded only when the message is forwarded.
Whether an email was printed: bug attached to print stylesheet
How long it takes to read an email: A connection that’s kept open, as pointed out by Forrest (this is also how Facebook tracks(ed?) whether or not you are online on chat).
Whether an email was deleted: Check If a message was read for a short period of time or not opened. In fact, they group “glanced” and “deleted” together.
Of course none of this will work if email clients disable images in emails.
EDIT: Here’s another question on this:
The OP actually has their tracking code, and this answer here explains how it works.