How does Litmus track their email analytics?

Question

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

Answer 1

One way I can think of doing that is having an embedded image that loads from a script on a server. The script would not return anything or maybe send data really slowly to keep the connection open. Once the email is deleted the connection would be closed. This way they could know how long the email was open. Maybe they just assume if it's open for less than 10 seconds it was deleted?

Another way is tracking the referrer - this would give a lot of data on what a webmail client is doing, but I doubt it would be useful with a desktop client.